It's convenient but there's still a tradeoff. Even once you get to South Station, unless you work right there you then have to have your secondary commute inside the city. I work in Kendall Sq so for me it'd tack on at least another 15 minutes. In contrast it normally takes me 15-25 minutes total for my commute as is. That's a big time savings.
I also have a lot more flexibility, for instance not needing to worry about catching the last train at night if I'm out with my friends or something like that.
For me, I'd rather pay more money for the proximity and flexibility but I agree that YMMV and that for others it makes a ton of sense to go to Providence. And really, I'd much rather go to Providence than a standard suburb because at least I get some of the benefits of urban life there.