i believe Alwil have to do their best with the next plugin release to have a better integration nomather what av solution used to be installed before that... (the key is supposed to be in registry settings for exchange, so during install it can be scanned and reported to give trouble or possibly give a sollution or solve it for itself even)
In regards to this, I see your point, and it has been brought up in the "suggestions for new avast" thread. I've seen people requesting a check of previous A/V entries before avast is installed.
Quite frankly, I don't think Avast is the only one involved with this. If you remove any modern A/V program, it seems to leave remnants behind. Usually, the program manufacturer produces a, "removal" program to get rid of the remnants. What I don't get though, is why we have to run a separate program just to remove the program, where an uninstallation program should have got the job done.
Avast however does provide the same thing. I can't help but think that if a user that doesn't know any better, uninstalls an A/V program, installs a competitor's product, faces problems, would automatically blame the competitor, and go back to the original A/V.
I can only assume that the A/V companies are doing this on purpose to keep their existing clientele.
All that I can say is that if you know what you're doing, remember to remove the previous A/V product with their supplied removal tool, or come to this or other forums for assistance.