Firstly, I want to say that this is a great forum topic. I think the answer to the proposed question isn't clear cut. If it puts over the product or talent that will be around when their temporary stays are over, it can be good for the WWE longterm.
If I had to make a prediction right now, I'd guess that Tensai/Bloom will be good for the WWE. He has seemingly never pushed his own product over that of the company, he seems willing to put anyone over, and I haven't heard anything bad about him. He's a hard working company man, solid in the ring (nothing special, but not awful), will hopefully rely on his manager or Laurinaitis for mic work, and hopefully will get one major non-title run that will end in putting over a deserving young face (ideally Kofi or someone along those lines).
Lesnar could go either way. He is usually willing to put over talent, but he's all about pushing himself and has shown that he cares very little for wrestling. While I can't knock him for not enjoying the hectic travel schedule (I probably wouldn't either), I can't imagine his presence and his inevitably huge push being good for backstage moral. He was physically a beast in the ring, but his technique was overrated in my opinion, and his mic work isn't anything special. Like I said, I hope he'll help, but I think he could go either way.
The Rock, in my opinion, had no effect long term, and I'd even go so far as to say that he may have hurt short-term. If Cena loses to Punk, Orton, Lesnar, or Ryback tomorrow, it won't mean as much as it would have before Wrestlemania. Long term, Cena will probably go back to the way he was after he's finished feuding with stars from the past, which is why I don't think the match had any longterm effects. Furthermore, with The Rock being gone and thus being unable to elevate any talent in the foreseeable future, no good came from that feud. Unless there's some kind of amazing story that Cena finds himself in that was spurred by this loss, The Rock's return failed in my eyes. I'll be glad if I'm wrong and something good does come from the Wrestlemania match, but I simply don't see how that will be possible at this rate.
As for the rest of the people you mentioned: HHH was never really gone and even had a couple of matches, HBK elevated the only program in which he was involved without putting himself over the product, and Austin hasn't really returned to the ring in any capacity.