Some people may decide to empty the paper shredder from the office & use the shredded paper as bedding. Not a good idea! Firstly the paper could have coloured ink, glue, liquid paper & other toxic stuff on it. Cavies eat paper. Would you want them eating paper with toxic stuff on it? Secondly, there may be the occasional stapler stuck on the paper. Sound dangerous to you? Also, freshly shredded paper can be quite sharp.
Poor moisture absorbancy.
Poor odour control.
Likely to have stapler, ink, dried glue, etc which are dangerous/toxic!
Shredded paper likely to be sharp (think of paper cut on your finger)!