I think it's one of those things where there is really no downside to using it. You need to give them that info (e.g., credit card) eventually anyway, so you're potentially saving yourself a step by doing it before you get to the resort. Once you get there, just pick the shortest line and use it -- there is no requirement that you use the "online check-in" line just because you've checked-in online, as others have noted. If you've not done online check-in, though, your only option is the regular line.
It might save you time, it might not, but there's no chance that it will unless you take advantage of it.