I'm one of those people who like big, fat complete one-book rulesets, so moving to the Runequest Deluxe edition was very appealing to me. I much prefer it's organization and format, although I miss the full-color monster stuff. I gifted the older books to one of my players.
Basically, for me it's just ease of access and portability....3 books instead of one. Also, no binding problems on my book, and it looks good; almost every other Mongoose book after this one that I've purchased either has a bowed cover problem and/or weak spine. I've also found most of their new books to be very, well, dark for some reason, as if the printing ink is set too high or something, but not in this book.