Some supporters of driver's licenses for undocumented immigrants are nevertheless disappointed about the mark because of fears it could be used as a red flag for federal immigration enforcement action (i.e. deportation). The bill's strongest supporters say, however, that even with this provision, potentially licensing over a million more people will improve safety and actually prevent undocumented drivers from losing their cars or even worse when pulled over for routine traffic stops.
See this piece on the ABC news website titled "Why Driver's Licenses Matter for Undocumented Immigrants."
For some enforcement perspective, both pro and con, read this second Los Angeles Times story here.
Finally, here is a fascinating story from the Seattle Times in June explaining that California is joining 10 other states in issuing licenses to undocumented immigrants.