The random position on the map you happen to be at has zero value, there's no reason to leave a contingent to hold that position. If you return to base, it's just as good a place as any to fight, and you can take advantage of the fact that the OTHER team just split themselves up, instead of getting split up yourselves. Wipe out the capper(s) and enjoy your numerical advantage for the rest of the fight.
Let's analyze this. There's two possibilities when you get the "base under attack" message:
1) They sent a lot or even all of their team straight to your base - The "lights rtb, everyone else stay" strategy just lost you the match. Congratulations. On the other hand, if everyone returns, the lights can stall the cap long enough for everyone else to get there. If their whole team is there, this is your ONLY chance to win. If it's just half of them, you probably secured the victory yourself when you wipe out that half with your whole team
2) There's only 1 or 2 light cappers - Ok, everybody returned to base and chased them off. You maybe even killed one of them before they got away. Nothing is lost, there is no important value in "holding the line" at whatever random location you happened to be at when you got the message. At worst, you are still even with the other team, at best you have a slight advantage now.
The only reason to not have everyone return is if you will sustain heavy losses from withdrawing (you're in a brawl), or you're close enough to their base to cap it first.
Edited by MuKen, 10 April 2013 - 03:23 PM.