Tamara, The reason that we have compact drive is that the gods at Shimano have decreed that it be speced on every bike that normal humans can afford. When such a decree is issued, it is rarely ignored. I'm sure it will make us all much better technical riders. Most of us would rather go slow than dismount, and we can't push those west coast 2x9 gears. The disadvantages deal mainly with wear and tear.
Smaller gears wear out faster than big ones.
It's possible to mix components to an extent. Most deraileurs handle some mixing very well. If you're a purist, only xtr is designed to work as a fully integrated non c drive group, and that is rather expensive.