Non-pumpkin advocates...
Pumpkin ale is my favorite of all beers. Not for all seasons, but I love it! With that in mind, I too came across this back and forth: to pumpkin or not to pumpkin.
So, I did a side by side of half batches. Using the exact same grainbill and hop schedule on both, I added roasted canned pumpkin to one of the mashes. I didn't throw any spices in just to truly taste if there was a difference or not.
Observation 1: The color difference is HUGE. The pumpkin-ed batch had much more orange/red in the final product.
Observation 2: The mouthfeel was noticeably smoother, rounder, and fuller on the pumpkin-ed batch. Probably not terribly different from upping the mash temp, except without additional unfermentables in terms of flavor.
Observation 3: The pumpkin added a slight... vegetal taste. It wasn't good and it wasn't bad. It was just there. It didn't smack you in the face, but it was very present.
All that being said, my experiment led me put 100% stock into the camp that a true pumpkin ale needs pumpkin (and spices too. My experiment may have tought me a few things, but straight pumpkin with no spice isn't as good a beer as with spices). The color alone is reason to do it. The fuller mouthfeel (which I believe are complex pumpkin starches) and the taste lend themselves to a great blend with the spices. You can go the "pumpkin-free" route, but I wouldn't.
(I'm sure somebody will question which scientific methods I used to test a difference. Nothing except side by side tastings with folks who came over and I wrangled into helping. They didn't know which was which and I asked them to tell me which differences, if any, they could tell between the beer. It may not be a double blind true scientific study, but it gave pretty consistent results).
As to when to add it, I've done both mash and boil. Mash has always been 10x more successful for me. Better mouthfeel and a whole let less of a PITA to drain!
Just my .02