Sorry for a slow responce - been off-line for a week or more.

I will say that I didn't have any problems deleting my /data/cache folder, but it does seem that over time it just creeped back up to the original size again - this did seem to take several weeks to get back to full size though, and I can't say that I had any issues. My lag is always much lower than what most people report anyway - combining that with the stability issues the xpac caused I'm not really sure what the root cause may have been.

My guess is that it slowly downloaded or unpacked some stuff as I went from zone to zone. So at most you could possibly save a little space if you just don't visit outlands (or maybe northrend after the xpac), but I'm not entirely sure that it wouldn't just download it the background anyway. This may also help with some of the issues installing to a SSD where you have plenty of room for the installed patch, but not 2x the space which is what seems to be required to install it.

Overall I don't think this hurt anything, but it didn't seem to help either. I'm back to 21.8GB for my WoW folder.