Apart from monster illustrations (and some minor illustrations for character races) I don't care for art at all in my rpg books. Maybe a nice illustration of a very important place in a campaign book, but I don't need pictures of weapons or guys in a chase scene. If WotC decided to strip all art from all non-monster books I would probably prefer this, since this makes for a much better text per page ratio and thus faster browsing/reading. I got my imagination for the pictures. And even better than simply eleminating all other art from the books would be filling the resulting space with vivid descriptions.
You're forgetting another important part illustrations play in a book, namely, to serve as "bookmarks" to help you navigate the text and reference the appropriate part quickly, and to give the eyes a break from the monotonous layout of a reference book.
Back to the topic:
I don't mind SOME art recycling. Say, for minor things, just to spice up a page of nothing but stats, you could use some item pictures from the Magic Item Compendium. Maybe you could use a piece to illustrate a paragon path or somesuch. Some Paragon Path illustrations are so generic-looking, you might as well have a recycled piece of art.
But there are things that ought NOT to be recycled. Major NPC/Monsters deserve their own pictures, based on the written description. If there's a previous, totally awesome picture of it, maybe you can use it again (say, WAR's picture of Tiamat from Complete Divine). It's just a matter of managing what gets recycled, and where.
And then there's the case of bad Photoshopped recycling, like the Human racial entry in the PHB, where a female tiefling was Photoshopped out and a human female was Photoshopped in, leaving a big white hole on the male human.
Good recycling -> Todd Lockwood's female drow in the MM, or Steve Prescott's Medusa archer from the cover of The Sinister Spire.
Bad recycling -> The aforementioned human entry, from the cover of Races & Classes, or the Kaz the Betrayer illustration in Open Grave, from the MM5's vampire entry.