why are they the most important?
The front are the most important to match for two reasons: First, soundtracks tend to have the most important information in the front channels (such as dialog), and second, because humans hear better in front of them then they do behind them. So, when a thing pans across the front, different timbre from the different speakers will make it sound odd. However, when things go front to back (or vice versa), it will not be so noticeable if the speakers are different because of the fact that things behind you sound different from them being in front of you. It is also more rare, occurring mostly in action films, whereas a car driving by in front of you is going to pan across the front, and that is common.
So, if you are on a tight budget, spend most of your money on your front speakers, because they matter more than the rear.
Ideally, however, the rear will be the same as the front. I use identical speakers for all channels (except the subwoofer, of course), and I will never go back to using different speakers for different channels, because things match perfectly this way.
