Rodriguez says proponents of bilingualism claim the following:
Kids who get instruction in their native language won't be so alienated and will be able to learn more in the very important first months of school. They'll also be more confident of their abilities and so they'll learn better.
Also, they will retain a sense of their "individuality" -- their ethnicity and their culture.
The question of whether they will benefit from it is a personal opinion. Both opinions make sense to me. It's hard to feel good about yourself when you don't know what's going on around you in school. But it also seems like learning English is the goal and immersion would work quicker.