Well...in most RTS games you can have a max of 8 teams..either 1v1v1v1v1v1v1v1 or 2v2v2v2 or 4v4. Like..if you played empire earth 2..you had an option to allow alliances to be broken or you cannot change teams. Giving players the options is better than just sticking with one rule! all alliances must be kept! its to boring that way!
But dont forget..games can be ruined because someone thinks it would be kool to break the alliance and go with the others...because then it could end up being 5V3. Its pointless...if your going to be on someones team..you should stick by that! Why would you want to break the alliance in the first place?? is it because your team is looseing and you dont want to loose?? if so..deal with it..and grow up!