Thousands of people have completed a bike trip (or more than one) across the U.S.A. and never needed a SPOT or something of that ilk. It's rarely needed if you're going to be on roads.
If you must have something like this to satisfy a helicopter parent/spouse/child, and it's the only way they'll agree to you making such a trip, go for it. Please do make sure they don't, as one mother did, go ape if/when your batteries run low. Just because you don't check in doesn't mean you're lying injured or dead in some remote ditch. You'll want to check your route ahead and alert them you may be off-line, or stationary, for a period of time. (One guy had to answer the phone to tell his wife he'd stopped for an ice cream cone; do you want to be like him?)