I have two choices, IMHO, as the third choice of standing in the check in line inside is not an option as that would be crazy since I have the other two options, which are online check in with only carry ons which saves me from ALL lines except security (then I wouldn't even have to do curbside which would be great but will never happen as I'll always have luggage to check in). Then there's curbside check in (whether or not I check in online or not I still use this feature since I always have luggage to check).
I unload my car, give the skycap my license and luggage and he checks the luggage and hands me my boarding passes and I tip him.
If I check in online and have my boarding passes already, I still have to unload my car and hand the skycap my luggage. I don't see it as saving any time. Yeah, the skycap has to give me my boarding passes but I'd rather he give them to me than my printer at home as that's one less thing to loose on the way to the airport.
It takes me five minutes or less with the skycap, unless for some reason curbside is busy that morning. I do have to go in the curbside check in line (if any) just as you do, only difference is you have your boarding passes already and I don't. It takes hims seconds to print them out and I'm out of there fast. This is what I prefer. I feel I don't save any time printing my boarding passes at home vs. the skycap doing it. That way he can tell me of any flight delays or gate changes that might have occured as well.
Whether I check in online or with the skycap, its all the same. I still have to check in with the skycap to give him my luggage. Once I tip him, I don't wait in any other lines except security just like you.
It's a matter of preference and what your used to. I see no need to check in online and then with the skycap again the next morning. I just do it all with the skycap. That is what I prefer. If someday I do not have any luggage to check, then I'd gladly check in online and skip ALL lines except security!! That would be GREAT but it's never gonna happen.