Purpose of using same speakers for surround?

Not one manufacturer has yet to produce a completely neutral speaker. Not only do different designer/manufactures deviate from neutrality differently, but even models from the same manufacturer deviate differently from each other. With that said; it is more likely that a given designer/manufacturer will more likely have more consistent design priorities, and even perhaps shared components within a given line, and therefore greater sound similarities. There is good reason to match sounds as closely as possible. Different variances from neutrality can add quite a bit of distraction from the semblance of reality. While perhaps not as important as matching left and right channels, the same reasoning for doing so, holds true for matching front and rear channels.

Not all multichannel music limits the surround channels to ambience. I have TACET CDs of chamber music which assign different instruments to different channels. I realized that I needed to upgrade the surround speakers when the viola had more robust sound than the cello.

All five channels are capable of the full 20-20kHz bandwidth, according to Dolby Digital.

IF they all sound similar, it makes it easier to tweak teh rest of the system for optimal performance.

If they dont', it may well sound good or not, just perhaps harder to tweak to get to a desired result.

For space reasons I have in-wall speakers for the rear which work fine. Plus I didn't spend a ton of money and concert DVDs sound pretty good.