Dude, that's the first I've ever heard that! Now, I'd say I'm an 80/20 DM/Player, but I DO play some reasonable amount of the time, though I must admit to having 95% DMed 4e, which is a bit odd even for me. In any case, I think DMing for other DMs is usually pretty good. I mean that way I get players who are practiced at coming up with plots and motivations, generally know what sort of things make a DM's life easier, etc. I guess the worst I've run into is a few "I'm going to be lazy now" DM types that didn't want to pull their weight as players. Maybe they didn't have the best players? Maybe they're just not really cut out to BE players, I dunno. So I can see where maybe now and then a long-time DM doesn't make the greatest player, but I think its more likely they'll be pleasant players at least than when you get Joe random player, who may have any amount of craziness under the hood!