That's sort of where I'm ending up. Not w/ electric vehicles, but with bicycling. I bike commute year-round & use it for most of my travel, but for the rare case when I need to use my car for longer trips it's nice to have. However, paying the cost of upkeep & monthly insurance for something I use so infrequently is starting to make less sense.
Space - Rear seats have been unbolted/removed in my SUV and a roof rack added for the climbing/camping/canyoneering/safety gear I bring. Perhaps there are rental places with roof racks, I've yet to see one.
Cost - Most rental places I've checked charge > $0.50 per mile if you are out of state or over a couple hundred miles (if they allow it at all). On a 3000+ mile trip I'd have $1500 in mileage fees. Don't be fooled when they say unlimited mileage - its like unlimited data for cell phones. If anyone has experience to the contrary where it isn't prohibitively expensive for a trip this length I'd love to hear it (keeping in mind I go down rough dirt roads so SUV/truck, not passenger car).
Comfort - Custom stereo and other things that you can have in your own car but not a rental. Also my car is modified so passenger seat can lay flat allowing one person to sleep while another drives, greatly reducing travel time.
Safety - I'm very familiar with the capabilities of my vehicle when I go down some of the rougher roads, or have to get through sand. Also how far I can push when the gas tank gets low.
I'm not opposed to the idea, it's just a hard sell. I'd love to have an electric car for commuting to work and just rent a few times a year that I need something bigger.
As a cyclist, I wish more of these services (Enterprise included) would have cars with bike racks on them. Car2Go did with their Smart-car fleet, but that was phased out.
You can sort of get this through Turo, but you don't get the network benefits (one-way rentals, easy replacement, etc.).