Personally I enjoy watching doubles.
I agree that for players who are plagued by injury, they probably shouldn't play.
But I think doubles is a good way to build up parts of a players game that are often neglected, i.e. net play and volleying which is rarely plan A (or B or C) for most of the top players playing singles. I think doubles has helped players like: Serena, Veus, Kim, Lindsay, etc, all build up more complete games.
For most players I think it is good to see them play doubles, especially in slams where they have a bit more time. It really shouldn't effect a players fitness, unless their are playing alot of 3 set matches. For most top player (looking to play 7 singles matches at a slam, which only 2 actually do) they are usually involved in a couple of 'gimmie' matches in the first 2-3 rounds, where they can win almost on cruise control.
Obviously it depends on a player personality, style of game and state of health going into an event, but plenty of top players do play doubles without any ill effects.