At Port Canaveral you quite easily walk from the parking garage to the port building. No shuttle needed (or available). You're probably confusing that with the rental shuttle process, or perhaps some other port.
On embarkation day it saves at least an hour of time I would have to budget for dropping off the rental car & taking a shuttle to the port. And on debarkation day it will probably save at least an hour of time that I would have to spend taking a shuttle to the rental car agency and lining up to collect my rental (no "skip the counter" feature at port rental car locations, & everyone is trying to get a car at once, unlike at an airport).
The hour+ each way does matter to me, as I'll be staying one night only at a great resort on both ends of the cruise, and want to enjoy each resort. Adding more time to the vacation isn't possible due to custody and work, so the time I have available is limited and precious. Add to that all of the hassle that would be avoided, not having to do those things. It's a vacation and I just don't want to deal with it. And I'll know that I have a car already, just in case I test positive. And as I mentioned above, I got an amazing car rental deal that doesn't cost much more than 2
DCL transfers between resorts & port plus two car services between the resorts & MCO (my original transportation plan). So lucky for me it all worked out.
No, if you have your own car, you don't have to give DCL any luggage prior to testing. At Port Canaveral, you will test in your car. If you test positive, you can just drive away without a wait.