PR by itself makes a great looking lawn. Dark green, with a shimmer due to the back of the leaves being very shiny. It stripes well, can be cut short, and is easy to grow, and grow fast. In a mixed lawn however, you run into a few problems that may or may not outweigh its benefits depending on what you want out of your lawn. PR tends to grow faster than the other cool season grasses. If you have PR mixed with TTTF or KBG, then the PR blades will be much taller than the rest of the lawn unless you mow often (2-3 times a week). If you are seeding an area with PR and KBG, the PR will be up and running very soon, ready for a haircut in 3-4 weeks, while the KBG will be lagging behind. The PR will outcompete the KBG for resources and shield some of the sun it needs to develop. Overseeding KBG into a PR lawn often proves to be futile, as mature PR releases chemicals into the soil that acts like a preemergent, greatly reducing germination.

I was confused by advice from extension agents and turf companies to plant KBG/PR mixes, not straight KBG. Per tiemco's comments above, this isn't a good idea. I didn't want to go thru all the work of planting a 15K sq ft lawn to end up with PR instead of mainly KBG.

I am no longer confused and actually excited to understand how a mix should be put together (ratio) and why. I hope this link helps others as well.

Here is a link that might be useful: Mixing Kentucky Bluegrass and Perennial Ryegrass