Hi Rich! Welcome to the group!
You're right. You could carry a spare gel-cell battery or two, along with a dedicated charger, and accomplish the same thing.
The thing I really like about the Jump-Its, however, is that during the winter, I can split them up, carrying one in each of our vehicles, so that we can provide jump starts to ourselves and others.
My wife and I have three vehicles between the two of us, so we need three Jump-its.
The latest one I just purchased had a built in air compressor, which will come in handy for emergency "winterizing" tasks ... we're soon headed for a series of mountain top campgrounds. I just checked the temp near one ... and it was 26 degrees this morning!
Also, being basically lazy, I fugure a Jump-It doesn't weight
as much as a regular car-sized deep cycle battery. I hate making my wife lift that much weight
when she can grab a fresh Jump-It and bring it over to the trailer!