I grew up in Boston and want to stay here. Plus, I live with my g/f who is currently in grad school in Boston. Additionally, I like my current school and only want to transfer if its really worth it. It is my gut reaction that it is a bonus to let them know that they are the only school I am applying for transfer, any opinion on this? I do mention it in my personal statement (which is basically a two-page essay in why I want Harvard specifically), but I have a fear that they wont see any competitive need to accept me if there is no chance that I will go to a rival.
My sense, based on, well, I don't know, is that Harvard probably isn't too concerned with its yield rate in general. I doubt they'll be too swayed by a commitment to attend if admitted. I don't see any way that it could hurt, though--I don't think transfer admissions are competitive as between schools.
Sounds like you have good reasons for only applying to Harvard, so I won't get on your case about it. Good luck!