I suspect that the process of debating and enacting the DREAM Act will actually increase the number of “slots” [for immigrants] by improving the public view of undocumented immigrants in general.
This is how politics works. If you want fewer abortions you focus on “partial birth” abortions. If you want legal pot, you start with medical marijuana. If you want universal vouchers, you start by focusing on vouchers for kids in failing schools. If you want to end the estate tax, you focus on the relatively small minority of families who are forced to sell off their business to pay the tax man. This kind of half-measure is not only much easier to enact, but it also tends to move public opinion to be more favorable to the 200 proof version. In an ideal world, voters would be perfectly rational and omniscient and we wouldn’t have to play these kinds of games. But they’re not, so we do.