This might sound stupid, but WHY do we need any kind of anti-snowballing in the nebula? Isn't the point that in those scenarios where it is used that the sides are basically even and its YOU who tips the balance into your allies favor? With miners vs pirates one, it even says that they are evenly matched, with some favor to the pirates, but you showing up changes that entirely. Your presence means that the balance is far into the miners favor. Yes this could theoretically mean you could just take out 2-3 bases in that scenario and the miners will win it for you. BUT its not going to be a quick win, and if you leave the nebula you lose out on all of the XP. So you want to stay and help them finish the job, for both the XP and the fact that the reward does have a time element as part of it.
Just you being their means your allies now have a very, very good chance at victory in those types of nebula. In effect, you ARE the snowball. If you do things right you assure their victory. And even removing the anti-snowballing factors doesn't just favor the player. It also means that if you screw up and die, and your allies start losing, you may very well have cost them their lives. They are gambling, and while they have a strong hand, misused it means they WILL lose.
I guess it's to prevent:
1. the fight for the first enemy base destroyed being difficult, and then becoming increasingly, boringly easy after that
2. to keep an early mistake from the player (aka, the allies losing a base first) from quite likely causing a inevitable, but long and drawn out, loss.
Now, whether those really are bad things, or just things that we "impacient, whiney modern gamers" have demanded for and gotten, is up for debate.
It may just be that this sort of snowball effect is a perfectly healthy way for a "game" to play out, it just isn't all that popular among many games these days. Though, PvP RTSs still thrive on this sort of early game snowballing type of thing, which I guess may be a strong argument for there to be no anti-snowballing measures. Maybe it is acceptable to say if you make a mistake early on where the situation is volatile, you deserve to lose unless you are so skilled you can put in the several times over worth of "work" to make up for it. In return, if you manage to make a strong early game progress, you deserve a much easier path to victory for the rest of the fight.
To be honest, I don't know which side I stand on yet. I can sort of see good arguments on both sides. Which is why I was advocating a comprosmise, not enough "anti-snowballing" to counteract losses inflicted (aka, you do still get an advantage), but enough so that a loss isn't going to put you so severely behind it will be extremely hard to recover.