For Merging AVI files I actually prefer a couple spin-offs of the VirtualDub ...
Nandub
VirtualDubMod
And if you like foreign films or the the trend to incorporate some foreign language parts into films then you may have to deal with non-embedded subtitles. I use ...
Jubler 4.1.2 for .SRT files (download 4.0 and allow a couple upgrades)
VobSob 2.23 for .SUB/.IDX files
SubRip 1.50beta4 to extract subs from DVD VOBs or convert .SUB/.IDX to SRT formats
NOTE: VobSub may handle .SRT as well but I prefer Jubler. Also I haven't used Subrip in a couple years, there may be newer/better freeware apps available for this.