One Piece 563

Well that ain't really working out for him now is it, seeing as how his crewmates were both persuaded and totally tricked into attacking WB himself right under WB's nose as he stood there "observing" the battle.
And now he ain't even observing anymore, now he's down fighting people, as he should've done from the start imo.

If you ask me he just stod there because it was convinient for the plot, as he couldn't get stabbed where everyone could see it if it wasn't on that high ground.

Bleh whatever.

Let's just say the war is like a marathon. If WB joins in right away, then they get off to a good start. But in the 2nd half, advantage swings over to the marines because marine big shots like Akainu, Sengoku and Garp enter the battle fully rested, and WB would be easier to bring down.

Bringing out your ace at the start of the battle is not very bright. A little common sense people.

We don't know much about Whitebeard's powers yet. I'm sure if he clapped his hands hard enough, the world may just break or at least the platform that the marines were stationed on. The author probably though that building up to the climax was the right thing to do as it helps prolong the life of the story and it's just signature shounen style. Somehow, Oda will most likely bring in more characters from Luffy's crew and have a huge brawl. They show Whitebeard being weaker now as well so maybe he isn't all that strong as he once was. Another good chance to show off Luffy's inability to leave others alone and his incredible Haki?

I agree with KLoWn when he says that it's just for the reason that he needed to be in view of everyone when he got stabbed. It's for dramatic reasons rather than for any in-universe strategy reasons, as Shiori also said. It's also true that you do save the best till last, as udou_akira said. I mean, if you accept that the pirates and marines are both using powerless grunts to fight beside their super-warriors (shichibukai, admirals, captains etc) even thought that's kinda pointless, it also makes sense that they let the lesser warrior scrap it out before saying the typical shounen thing of "Right! No more holding back! Time to get SERIOUS!" It also sets the battle up as Whitebeard vs Sengoku, like L vs Light trying to outwit each other.

To be fair, though, he did cause a huge tidal wave which, if not for Aokiji's response of freezing it, would have been a pretty devastating attack. Although what KLoWn said is the real reason for it, as well as just "the rule of cool" meaning that it's better for him to just stand there observing things, what's happened so far would probably only have taken a few minutes. Many of events would be going on at once but if you've been reading this arc over many weeks and months it'll feel like he's been standing there forever probably.