I use a Kindle for ebooks, and use Calibre to convert the epubs from my subscription to mobi format. Unfortunately it's got limited capacity.
Today I went through and stripped the advertising pages and graphics out of the original epubs of the Pathfinder novels using Sigil, then reconverted to mobi, and put them back onto my Kindle. I gained 68 megs of space.
I'm not bothered by the advertising per se, but I thought it might be useful info for anyone else running short of space on their digital readers.