The turning point for me was that my Sprint FrankenPre 2 just wasn't cutting it anymore. It's hacked together hardware and software, I didn't expect it to hold up forever. I also had a Pre3 on AT&T, and unsurprisingly I was using it much more than a Pre 2 on the slower Sprint network. So I had to ask myself, "Why am I doing this?" I've been a Mac user for years, and iOS 5 was finally an iOS I'd be satisfied with using full time.
There's a lot that webOS does well, and there's a lot that iOS does well and Android does well and... well, BB10 and Windows Phone have their strengths, but I wouldn't say they're up to the level of challenging iOS and Android with the "must have" level of features. At least not yet.
I still have and regularly use my Pre3 on AT&T - I carry it and the iPhone in the same pocket practically every day. Whichever one gets an alert first is generally the one that gets my attention. The Pre3 usually comes in ahead on Google Voice text messages (excepting iMessage, of course) and calls, while thanks to the excellent Gmail app the iPhone gets most of the email reading and sending. iPhone gets 100% of my Twitter reading (TweetBot) and 95% of the sending, while I manage my Google Reader feeds about 50/50 between both using Feeder and Reeder.
Right now I've got a Nexus 4 sitting on my coffee table waiting for my attention and I might be picking up a Z10 soon as well. My SIM card is likely to start traveling between devices, though I'm certain I'll miss the feel of the Pre3 in my pocket when she's not carrying the AT&T signals.
For what it's worth, that' my very nonstandard experience. Most people don't have two phone lines, let alone two personal phone lines, so they don't have the luxury of saying "I'm going to have a Pre3 and an iPhone!" We all have to make our own personal judgements on whether or not we're going to switch, when we're going to switch, why, and to what.
But don't let loyalty to webOS hold you back. There's no place for loyalty in smartphones, especially do a platform with a future that's in question. Smartphones are tools - pick the one that best gets the job done for you.
It never hurts to look.