Hi, rog. We've used whiffle balls for years - mostly the golf ball sized ones. We have a small yard and can't always get to the field. Of course, these days he hits them over the house and into the neighbor's pool all the time.
We even had one of those little whiffle pitching machines that you can get for $200. It was a lot of fun. My feeling is that they certainly help build hand-eye coordination and your son can practice his swing mechanics any time and get in a lot of reps. We have one of those SwiftStick's too. It's like a broom handle with a foam "barrel," and you use it with the golf-ball sized whiffle balls. Because it's light, my son can get in a lot of swings and not get tired, and he can really tell when he doesn't hit the ball on the barrel. It hasn't hurt his mechanics at all. In fact, I'd say we've been very successful. He made the varsity HS team as an 8th grader, primarily because he developed a very nice, natural swing and can hit varsity pitching.