The problem with PDFs is that they make a lot of assumptions about layout and formatting, whereas mobi and epub are HTML based, and therefore work on a much larger array of devices - from mobile phones to Kindle for PC, including, critically, eInk based readers which are way better for sitting down to read than anything LCD based.
Though if you are writing a book about design you often want the level of formatting and design ability that a PDF gives you. It's hard to give that up with an ePub.
