So if you buy a mammy, just make sure that you do more damage overall than you take. As long as your team ends up with more points after your vehicles use (or destruction of buildings of course), I'd consider it worthwhile. All it takes is to destroy a couple of stealth tanks. What kills GDI is if 3 or 4 engineers are repairing the mammoth. That just feeds the enemy's bank/point total, so it's not worth it unless the mammoth is doing a lot of useful damage. Typically though.. several enemies are attacking the mammoth so it's taking more damage than it's delivering.
On another side of this.. hunting down snipers and raveshaws with a 500c sniper can be quite rewarding.