Do you pay a registration fee for your boat? Here in NH, cartoppers (which means pretty much any boat without a motor) do not have to register with the state. That means they pay nothing for the boat launches and the parking areas. I have to admit I get a little bit steamed when I see cartoppers blocking up a State boat launch. In my opinion they should make efforts to only use cartop loading areas that do not have a concrete ramp, but obviously I would not ever say something. This includes people that have thier kayaks on trailers- they don't have to pay a registration fee. And to make it even worse, if they don't fish and buy a license, they also don't pay for the Conservation Officers that need to rescue them when they get in trouble. Anyway...
If you indeed pay a registration fee for your boat (if not now, I'm sure you will when you add a gas motor) then you paid for that launch and deserve your time. Or at least I would guess that is how it works in other states. So don't feel guilty about using it. When I had my squareback canoe (which was registered for a gas motor), I ended up buying a cheap trailer for it. Before I had the trailer I would make every effort to launch at cartop only lakes and areas, even though I paid as much for my canoe as an 18' Bayliner with a V8 on a trailer. But on the rare occasion I was cartopping at a busy ramp I would dump the boat then get it off to the side as quick as I can, then get my truck somewhere out of the way and hump my gear down to the water.
To be honest, there are some people even with trailers that can take an amazing amount of time at the ramp. So just do the best you can and as long as I see someone making an effort to expedite thier time at the ramp and be efficient, no worries.