I don't have a prep sink but certainly I'd want soap available if I had one.
If the sink is mounted in an island, I might opt for a squeeze bottle that could be stored away instead of a permanently mounted dispenser.
I'm sure those with prep sinks will chime in to give you their thoughts.

i couldn't fit one in. i have an instant hot at the prep sink along with another hole for my pop-up drain and side spray. i was willing to lose the side spray, but my faucet won't operate without it. i figure i'll live.

I have a soap dispenser (SD) at both sinks. In our case, we have hand soap in both...it's what works for us. Most people here, though, put hand dish detergent in theirs...and that would be useful at both sinks.

For us, hand soap worked b/c I like to have my dish detergent more flexible...when soaking, cleaning, etc. dishes/pots/pans, I like to be able to put the detergent exactly where I want it, in any part of a dish/pot/pan. When it's in a SD, you're limited to only putting it in the edge of a pan and you have to take the item to the SD instead of taking the detergent to the item.

A side benefit is that it's cut down on how much hand soap we go through b/c one pump is all you really need.

Thanks for the input. I'm already putting the Kohler Karbon at both sinks and the thought of drilling yet another hole in my marble is making me anxious.

The prep sinks at my island.

What to do, what to do, what to do.......

You know, the Never-MT I got for that prep sink hasn't worked, so I just have a lovely bottle of scented hand soap at that sink. And I don't miss having dishwasher soap there, after almost two years.

I have a dispenser with a Never MT at both sinks. My instant hot is at the main sink -- closer to our coffee service and handy for soaking stubborn dishes. I would suggest a soap for hands or both hands and dishes at the prep sink. I use my prep sink more for rinsing and hand washing than dish washing. You can find gentler soaps you can use for both, and for hands you generally only need a tiny dot of the concentrated soap, not a full squirt.

I put one and mine. Love it.
I sure wouldn't want a bottle of anything sitting there.
I use mild soap that can be hand or dishes.

I have a soap dispenser with NeverMT for hand soap at my prep sink and love it. (Donna - did you try taking out a little soap from the bottle and adding an ounce or so of water? Some soaps are a little to thick for the NeverMT - we do that with ours and it works fine after the soap is slightly thinned.) If my hands are dirty, I can dispense by pushing with my wrist or the side of my hand. Our prep sink is on an island and we wouldn't want a bottle traveling around on the sink top.

I like to be able to wash my hands right away after handling raw meat at the prep sink or washing dirt off produce. I don't want to drip across the kitchen to the main sink.

Our prep sink is also at the edge of our kitchen near the kitchen table so it is a convenient place for those entering the kitchen to wash their hands and for after meal washing hands and faces for the grandkids.

At the main sink, I use a bottle of dish soap because I want to be able to squirt it on things I'm washing anywhere in the sink. A bottle that can move to the item works better for me there than moving things to the bottle.

We have a single hole faucet, an airswitch for the disposer and the soap dispenser and it doesn't look cluttered.

We have the same three items at our prep sink and I was a little concerned about a cluttered look, but I think it looks good. We have soap dispenser on one corner, faucet on the other and the air switch near the faucet.

At a prep sink, I would want to have fruit / veggie wash and hand soap available. I currently do not have a prep sink (maybe some day when I get to my kitchen remodel), but I would use it for cleaning fruits and vegetables. Additionally, I often find myself needing to wash my hands while prepping a meal (raw egg gets on the hands, etc), so hand soap would be important.

"...already putting the Kohler Karbon at both sinks"

Me too on the Karbons! (:

I'm also concerned, I would love the convenience of the Karbon, Air Switch, Soap Dispenser AND a hot water dispenser...but I'm worried about that many items cluttering up one prep sink. My prep sink is along the wall, not in an island, but in full view. I was just about ready to give up the soap dispenser and just have one at the main sink, but after reading this thread now I'm not sure about leaving it out.

If it helps, air switches don't really contribute to "clutter" b/c they're pretty much flush w/the counter.