Are there any downsides to using white printer paper as background for product photography?

Actually, printer paper is not true white. It's produced to fool our eyes in thinking it is, which sounds weird I know but it is. You are probably getting more transmitted light/reflection from the printer paper as it at has a small amount of gloss on it. Photographic background paper is completely matte so it will give you a slight exposure change vs printer paper.

I've used cheaper papers before, especially when first starting out. Those $120.00 rolls can be expensive at times. It can work, but I found that it often would result in uneven tones and the texture did not produce a smooth effect compared to the photographic stuff. Also basic printer paper is no where near as durable as the photographic paper. It will wrinkle and tear far easier and you end up going through a lot more. Also, there is no guarantee that two rolls printer paper will be the same color temp. So if your shooting catalogues you could end up doing a lot more post to balance the backgrounds.