if they pull off well.
In a lot of ZvP matches (I play Zerg), the meta game tends to be early Nexus. Instead of playing the macro race ( i get two expansion), get the early roaches and do as much damage. If the toss only has one canon and one sentry, this is an auto win. At least i will inflict enough damage for me to get my own expansion.
If you look at 7 Roach Rush play -> http://wiki.teamliquid.net/starcraft2/7_Roach_Rush_/_Expand_/_Lair, you get an exp right after. So those who stay in one base are just bad players.