For us, online check-in's real value is psychological...in two ways.
First, it makes the upcoming trip seem so "real." It's like a head-start on it. So I always try to use it.
Second, while I've never arrived at
DVC (or any hotel, for that matter) and discovered "no room at the Inn" despite having a confirmed reservation, I'm aware that can happen. Checking in ahead of time is (psychologically-speaking--probably not actually) feels like a way to minimize that risk...
There are times I can't use it--the system for a while didn't allow it with linked reservations, and there was another glitch a while back that prevented me using online check-in. No biggie.
For us, there's never been any "actual" advantage to using it...just the psychological advantage. Whether it's faster or not just seems to depend on circumstances. I've been there at times when the online line was longer than than the line for people actually checking in. And it makes no difference at all if you're looking to make any changes to your assigned villa. And I've noticed no impact at all on how fast our villa is ready. Sometimes it is, sometimes it isn't. (We almost always arrive between noon and 1).
I never give the reservations people (online or at the front desk) my credit card anymore. That doesn't seem to make any difference either. (I'm fine w/using the credit card, rather than the room key, for purchases and if the front desk doesn't have my credit card, they can't mess up room charges...time to sort out such charges (happened fairly often w/"free" internet access back before WiFi) on departure day is always a problem...problem solved by simply not giving them a credit card...
Still, even tho I don't see any "real" advantage to it, I use it when I can because it makes me feel good, sooner, about my upcoming time at WDW!