Collusion! Nash Equilibrium! MAN, I'm glad I had micro-economics this semester!
Well I'm going to have to hope over with phrontist on this one for the simple chance of an intelligent argument (because its no fun when nobody's arguing
So the higher ranked sister team has been doing great all day, is 1st seed, and comes into one of the last matches of the day with you. Their alliance is them, and two robots that have had some "problems"

we all know how that goes) and your alliance consists of you and two other average to strong robots. They can almost, I mean ALMOST, beat all three of you by themselves, but it comes down to this:
dum dum ... dum dum (does that sound like a heart beat to you
if you go up onto the ramp, your alliance wins, if you go play "defense"

they win (and you know they are going to pick you as their first pick, and their is a very strong chance you will win your division at nationals and have the best shot to win nationals)
so what do you do? huh?
ps. sorry i had to make this so hypothetical and far-fetched, but this is the only way i could stretch this to make you, the average FIRSTer, even consider it

... let the intelligent punditry resume!