I don't think that it really gives you that much of a bump. Look at my app for example. I have a competitive mcat (32R), and a less than competitive gpa (3.36, with 3.4 bpcm).
I do have some very unique ec's, did very well at a postbac, have good letters and all of this, coupled with growing up disadvantaged, has resulted in 4 interviews so far. I don't have 20 interviews, so I feel it is safe to say that selecting this option was no magic pill. To not select this option, in a situation like yours, is like holding back information. Let me be more specific.
In my case with BU, they are the only school that I am sure had a strong interest in the fact I grew up disadvantaged. This is not because they felt sorry for me
Rather, they are an extension of a public/private partnership and place a greater emphasis than most on treating the indigent population. As a result, they had an interest in an applicant like myself, who is likely to not be turned off by treating the poor (which is true in my case). If I didn't select that option, they may have thought less of me and not offered an interview. As a result, I would never have visited this school which left such a positive impression on me.
The moral of the story? Schools are looking for good fits, they are not in the business to give handouts. It is in your best interest to give them as much information on your background as you can so they can determine if you would match well with their beliefs.