I actually think easy swimming helps me recover from tough runs. The other thing i think it can do is prevent achilles tendon problems. Runners have really inflexible ankles swimming helps this, especially kicking.
i've been swimming 2-4 x per week 20 or so years now in addition to running and i have never had achilles tendon problems.
The downside to swimming is that like biking, to get the aerobic benefit that you would on a run, you have to work really, really hard to get your HR up to "easy run" HR. And you really, really feel those running miles when you try to swim hard.
At one time i was running 70-80 and swimming 3000 yards 3x a week. I don't know if it helped or hurt but i have never regretted suffering through masters workouts to learn all of the strokes well enough to get a hard workout when i'm not trashed from running. i have been really happy to have swimming as a sport when i've been injured.
I figure when i'm 80 and can't run anymore, i'll still be able to get a great workout in the pool.
in short, i say do it.