I have an old MBP A1150 I inherited from my B-I-L. The swelled battery was still usable, however, I sometimes had to power up the book several times to get it to boot. Eventually the battery top bowed/separated and pushed up on the track pad and enter bar from underneath to the point where I could no longer click on a function. Had to take it out and run on house voltage until a replacement battery arrived. At least I could still use it. Once I removed the battery, the enter bar would depress far enough to function again.
So, if you aren't replacing a swollen battery, you might get by for a while, but you will get to the point where have no other choice but replace it, once it swells to far/wide/high for the keys to function properly.