It is very important. It directly effect the munber of MP's you have which translates into how many shots per turn you can get off and the causalties you cause.
"Unit quality seems VERY important, not numbers."
Number can be important. There is nothing worse then firing your last shot half way through the fight

Building high experianced full strength units takes a long time but it can be done.
There are basically two ways to get an 80+ experianced unit. You can try to limit the number of losses a unit gets in a battle so when you add replacements the experiance doesn't drop much or you can use the disband feature to combine two highly experianced units. If you can get a core group of full strength 80+ experianced units you can really kick butt.