I love the idea of a griddle. But whenever I have used one, I discovered the same thing you discovered. It's not as useful for my style of cooking. The good news is that you could commandeer it to become your simmer burner (i.e. a poor man's French top).

But in the end, for our kitchen remodel we made a conscious choice to go with a four burner range and two small 24" wall ovens (CSO and SpeedOven). Couldn't be happier with our choice.

It just depends on how much you love using a griddle.p, and what you are accustomed to using.

Our old Jenn Air cooktop had a griddle "attachment" that covered two burners' space. I used it maybe three or four times in 18 years before we got a new cooktop. It was heavy and bulky to clean and I was really happier making pancakes in my cast iron skillet.

I think the part that really cured me of the notion of needing a griddle is the discovery that the star-shaped burners in my gas range have a really even heat distribution. Unlike traditional capped burners, there is no change in flare when power is adjusted, nor is there a permanently cold spot in the center. This sounds like a minor thing, until you realize that it opens up a whole bunch of new options. I can now use thin rolled carbon steel cookware. Normally, that's an awkward material to cook with, as it doesn't distribute heat very evenly. That's why most people end up buying heavy multi-ply cookware for their ranges.

But if you have a range that can use carbon steel, you get most of the advantages of cast iron combined with very fast preheat and recovery time, and with a lot less weight to lift.

I find that most of the things that I would normally want to use a griddle for, I just grab my cheap Lodge carbon steel skillet. Or if I make larger quantities and I am concerned about splatter, then I use my 14" round-bottom wok with its convenient tall side walls.

That's not to say that a griddle wouldn't be nice. It is. I have used griddles before and when properly preheated, they are a joy to cook on.

But honestly, 90% of the time, I'd still use my basic skillet.

A good thermostatic griddle is predictable. It maintains somewhat even heat across its surface and over time. If you set it to 340°f then it will stay within about 20° of that which makes for better results and makes cooking more enjoyable than having to constantly adjust burners or deal with something that got over or under done.

A burner top griddle is uneven across its surface and even if you are extremely good at constantly adjusting the burners it will vary quite a bit over time.

That said, if you only use it once or twice a year and aren't picky about the results then burner top might be good enough.

While we user ours as a griddle for pancakes and stuff, we more frequently use it as a flat top to prepare sauces and keep them warm or to sear meats or whatever. So, contrary to what @barryv_gw said, it's actually easier to go the other way. The flat top capabilities are much more useful and versatile for us than the four burners we give up and we still have four burners besides.

As I have said before, I absolutely love the promise of what a griddle can do. It's really fun to cook on, when it's all set up.

But in practice, in order to work like this, you need to preheat the griddle for a while, so that it reaches its equilibrium state. On the other hand, my thin carbon steel skillet is usable after a mere seconds on the stove. Yes, I need to manually adjust temperature, and it would be nice if I didn't. But its almost second nature at this point. So, the fast turn-around time wins every single time.

Similarly, at the end of the meal, I take my skillet and run water over it. A quick wipe with a paper towel on the stove while I heat it up one more time, and its cleaned up. Takes about half a minute. With a griddle, I find it is always more work to actually get it cleaned.

In the rush of preparing breakfast and packed lunch for the whole family each morning, those extra minutes count.

On the other hand, if I had time to plan a little more, then yes, a griddle would be wonderful. And having it do double-duty as a flat top is certainly attractive. So, I guess it really depends on your cooking style. I do have time for long elaborate meals every so often, but the bulk of my cooking is very reactive and has to be fast.

When will you ever use 8 burners, though? Do you even need 4 at once for any but the most complicated of holiday meals? I never do, but you have to evaluate your own cooking style to make this decision.
I personally love having 4 burners and a double griddle. I can bust out a full batch of pancakes in a few minutes, make a bunch of quesadillas, grill up sandwiches for family/friends all at once instead of 1-2 at a time... it's so convenient and such a time saver if you like to eat with your people instead of being stuck at the stove while people eat in shifts.
I turn on the vent hood and preheat the griddle before getting ingredients out and ready, but it doesn't seem to take long at all to be at temp and ready to go, just a few minutes. It takes a loooong time to cool down after use, but it's easy to clean!
We bought the Wolf dual fuel range with 2 burners on either side of the double griddle. I worried about the configuration, but it's actually nice to have an additional 24" of workspace between the two sets of burners when the griddle isn't in use. I have no regrets and would hate to go back to an all-burner range.

Their are a couple of advantages other than flexibility of your burners to having an add on griddle. Very few built in griddles have an aluminum plate. Aluminum allows much more even heating. It heats more quickly and changes temperature more quickly. The usable space on an add on is about 50% more than a built in over two burners.

These are the ones I would consider.

Royal industries has a bare aluminum one and the same in nonstick. I have the 15”x23”. The bare one seasons over time.

http://royalindustriesinc.com/index.php?main_page=index&cPath=4_22_48

Also this one in steel seasons like cast iron but is heavy and does not heat as evenly as the aluminum one. They have a four burner size but I couldn’t lift it.

https://www.dvorsons.com/ChefKing/Griddles.htm

I have a 48" Bluestar and opted against a built-in griddle (and also the built-in grill) in favor of eight burners, which I do use often, and not just for holidays or when canning.

I just don't use a griddle often enough to justify the valuable real estate in my kitchen. It all depends how you cook.