I used a wind shirt and a poncho on the PCT, didn't carry a rain jacket, and this was a good combo IMO. Factor in that for much of the PCT it doesn't rain much; in 2008 I walked in virtually no rain until Washington state.
The nice thing about a wind shirt on the PCT is that it can be a really good layer to have for temperature control on windy days, whereas I think that sometimes a rain jacket might be too warm (and sweat-inducing). I actually mailed my wind shirt home along with a few other things from the first place that I could (Mt. Laguna P.O.) and literally the next day it was crazy windy, so I made myself a substitute out of a large black plastic lawn trash bag. Good enough, though sleeveless. Until I could get my wind shirt mailed back (!).
The nice thing about the poncho for the first 700 miles was that it was my only shelter as well, and this worked fine; it can rain (hard) there, but it's pretty rare, so this helped offset some of the water weight one carries there.
In the Sierras a rain jacket might be nice, but in fact I stuck to wind shirt and poncho and I think I was happier with wind shirt there too. Not much rain in the Sieras for me, and even in the snow you're working hard enough that a wind shirt (and sometimes not even that) were typically fine.
Different trails require a different mix of gear; I liked having a separate pack cover (actually, a Packa) on the AT, and plan to use the Packa on the CDT, but for the PCT I think a poncho is a good option, along with a very lightweight windshirt.