Your point #3 about tanks wasting time is totally flawed. For every tank you have shooting at a building, that's one to two engineers or a Hotwire/Tech that IS NOT albe to help their attack because he's gotta defend that building or leave it. One Mammoth can keep 2 or 3 engies occupied, especially if you're spreading your fire to multiple targets. The fact that there are less to attack you means your teammates can theoretically take the advantage.And about snipers: they should be given more points for taking out repairing engineers I think, but the points from being a sniper comes from taking out expensive stuff, just like anyone else. You kill a 1000 cred unit, you get about 93 points, if I remember correctly. That's a fairly decent amount of money for you too. Vehicles, same thing, you get good points for dropping an Arty/MRLS/Hummer/buggy with a sniper.
It's all about killing the bigger threats to your base. An engineer repairing a tank is much less of a threat then the actual tank itself, although the tank is much more of a threat with an engy.