Solo the Wingman
*smooth jazz*
I don't think Philip coming back really hurt the ending. He and Shotaro are meant to be together, they're partners, 2 in 1 and all that - it is the point of the show. I think a lot of the complaints about it are just because so many people think character deaths are cool or somehow make the show better
Personally, and I assume that this is the way that most people think, it's not that character deaths are cool, it's that character resurrections cheapens the sacrifice of that death.
Most of the time when a character 'dies' in a show, that impending death is built up and given great importance-- especially when that character is making the ultimate sacrifice to further the goal of saving the world or protecting someone else. When that death is undone in the wrong way, it negates our emotional investment in the scene/series and cheapens what was supposed to be a great sign of heroism and selflessness.
Take Date's 'death' in OOOs. The first time you see that scene, it's a powerfully emotional experience that causes us to resonate with Goto's grief and resolve. It's that impact that sets up the awesomeness of Birth's rebirth.
When Goto pops up and goes 'Just kidding!' later on, the emotional impact of that scene is forever lost. On repeat viewings, you're never going to get quite the same sense out of Date's death because you know he's just faking it.
Imagine what Gurren Lagann would have been like if Kamina had reappeared halfway through the second arc. A literal ton of character development and emotional investment would have been completely undone and the core of the series would have been destroyed.
Now, I have to confess that I never saw the final episode of W. I was moving at the time and never quite got around to downloading that episode. But if Philippe died in one episode and was resurrected in the next, I could see others' frustration. It really does make cheapen what was supposed to be the defining moment of a character arc.
Edit: ninja'd.