I also think cutting tubes is a perfectly valid strategy. Doing it late game is certainly no worse than using a nova to a CC, if even that. They should just be on the lookout for that. Doing it early game seems fair too. If they have an earthworker early game, it's quite possible you have one too (say in a land rush game). In which case rebuild any tubes they destroy, or build extra before they are destroyed. They can cut at most 3 of your active tubes before their earthworker is destroyed. They can't eactly repair them early game either, making it costly, especially if they need to send more than one. Plus, they are slow so you should see them comming in advance.
GP rushing is definately cheap, but convecs are also slow. You should see it coming. Unless your bases are really close, you should have time to put up GPs of your own before they get there. Granted, there is the issue of not knowing where they will place the GPs, and once you place yours they are immobile and so can be avoided. Although, that is somewhat dependent on the map. If placement is an issue, you can try stalling them long enough to get mobile weapons platform, which shouldn't be far behind. Maybe follow their convec around and place your GP right after theirs. You probably won't win the fire fight, but you'll at least keep their GP busy for a while. GP rushing isn't something I'd like to see way before combat units can be built, but I see no reason why people shouldn't try building GPs in an enemy base anytime after combat units are available.
And sure, some of those rushing tactics may be a bit on the rude side, but at least it ends the game early and you didn't just waste hours playing a game you didn't enjoy. Personally I'd love to see more creative ways of playing, other than the standard rush. If it ends the game early, then no real time wasted. If it's a long game, then at least it wasn't the standard rush. Besides, how many of you find at least a little amusement in some of the things people have tried?