1. It's a shitty UX
2. There are more people without an OAuth provider than there are with them
3. It's a sure fire way of killing your conversions
4. It means people start getting tethered to providers
5. It's very complicated when it goes wrong
6. THIS DOESN'T SOLVE THE OP'S QUESTION AT ALL. OP POINTS IT OUT. YOU IGNORE OP.
Enabling SSL stops people sniffing sensitive data on public wifis. That's why everyone says enable SSL by default.
Also there's something wrong if you're a programmer and can't afford an SSL cert as it's the same price as a couple of beers.
I also find your password advice extremely questionable, it just doesn't make sense to me.