sudomod

As long as i don't do anything stupid is there any reason that I can't use a mobile phone battery to power a Rpi? Ive got a few Nokia bvt-5c batteries these things are 2500mah (3.8v 9.5wh) and only 5mm thick. As we are always on the look out for high capacity small form factor batteries for portable pi projects they seem like an ideal solution, on top of that if the phone(s) that used them are no longer being produced they are dirt cheap for the capacity and yet I rarely see them used in pi projects. The Arduino guys dont seem to have a problem using them.
Is there something i'm missing?