If it were possible, every team would have a platoon at all 3 outfield positions. But there has to be a value in an everyday guy, too. I want to see his power develop, and the starter rarely goes 9 innings. If they are a suitable platoon, great.
I don't think so. It all depends on your personnel. You only have a limited number of roster spots, so I don't think carrying 6 OFs simply to platoon makes sense. However, it does help where your 3d and 4th OFs show big splits (certainly Willie does) and you can take advantage of those splits to get above average play out of a combination that is, individually, average, below average, or limited in durability. A guy like Willingham hits righties very well. You would not platoon him. Morgan is so good defensively that you keep him out there most every day, but I'll bet that when he does get a day off, it'll be against a tough lefty and Taveras will be out there.
J-Max was getting it done today. Slick D, and had a little pop. 
Very happy too. I want to see him work out.
I'll admit I'm coming at this to maximize short term performance. I can see the argument that, this team is going nowhere, that 65 - 70 wins would be the right direction, that we need to see if J-Max is the next Mike Cameron or Jason Werth, and that we will not know until we try.
I/M/O, I think a Willie H / J-Max platoon will be worth 10 - 15 runs above average, produce 0 - 5 runs above average, and end up being worth about 3 - 4 WAR. That is very good. Among NL RFs in 2009, Only Upton and Werth produced more than that individually.