The vast majority of home owners use a mix of seed types to accommodate environmental factors and specific objectives they want to achieve in their lawn care/usage. Also, monoculture (one grass species) is not recommended, as a disease or insect infestation could wipe out your entire lawn. Spreading (rhizomes) type grass are not always the correct choice for home owners. For example, if you have a high traffic area of your yard, you will probably want to go heavy perennial ryegrass. Heavy shade, that's going to require a fescue mix.

KBG is the dominant spreading grass and with the exception of a few new cultivars, it generally has shallow roots and poor nutrient absorption, which means it is expensive to keep green. Lots of water and fertilizer and sun, which not every property or budget can provide. The idea is to get a nice mix of seed types that achieve all of your goals. If you have full sun, high budget, and irrigation system to maintain a full KBG lawn, then go for it. However, there are trade offs. The extra time you spent watering and fertilizing, the fescue owner will have to spend overseeding.

SC77, I wish there was a "like" button for your post!

I read the OP's question with interest and your reply is very helpful. I have been recently wondering the same thing as the OP, and thinking about adding in more KBG to my fescue lawn, mainly because of the intensity of fall work on the fescue, due to thin and bare spots. And those weeds! I always hear how the best defense against the weeds is a thick turf, but it seems you can't get a thick fescue turf without annually adding seed.

Kentucky Blue Grass, many of the Fescues, and Perennial Ryegrass are all cool season grasses that grow best in the northern areas of the world. These grasses, however, can be grown further south but will be under stress and that is the major cause of plant diseases and insect pest invasions. Talk with the people at your local Cooperative Extension Service about which grasses will grow best where you are.
Since the soil any plant is growing is the most important part of that plant the soil any turf grass is growing in is just as important. Providing a good, healthy soil for grasses, proper soil pH, balanced amounts of nutrients, will allow those grasses to grow and spread much better than soils that are less healthy.

Tall Fescue is the grass mostly used here. In fact, much to your dismay I'm sure, the vast majority of grass grown here is good old K-31. The problem with Kansas is that it gets colder than Massachusetts in the winter, but almost as hot and dry as Texas in the summer. So basically there is no good option. Any warm season grass would die in the winter (maybe something super expensive like Yukon bermuda could make it). KBG and PR will have to be watered excessively in the summer and probably need some mild shade as well. That leaves TF. Still not ideal for the summers here, and it doesn't spread, which just for me personally is rather annoying.

Maybe zoysia is the middle ground. Not sure if it would make it through the winter or not. I haven't really looked into it much.

Talk with the people at the local office of your Kansas State Universities Cooperative Extension Service who have many of the answers to the questions you have because you have paid them to do the research.

The spreading type grasses all go dormant and turn brown in extreme cold and extreme heat/drought. Fescue and rye do not have that trait. They remain green all year even when they are not growing (dormant).