Here in New York, from time to time I'll still spot broken safety glass in the gutter. Sometimes the burglarized car is still sitting there, the seats picked clean, the glovebox open. I'm amazed anyone in NYC would leave anything in their car to tempt a thief, but I think our declining crime rates are making people complacent.
For those who live in areas where burglary isn't a problem, a car can be a handy place to store things. Off-road vehicle accessories manufacturer Smittybilt makes a line of gear for just that purpose: Their G.E.A.R. seat covers use what looks like the military's PALS (Pouch Attachment Ladder System) webbing to modularly attach a series of bags, pouches and tool rolls.
The bulk of the G.E.A.R. line-up is made to custom-fit different models of Jeep (CJ, Wrangler, and Wrangler Unlimited models made from the mid-'70s to today), though they also offer Universal models without the seatcovers.
While I like the concept and design, they may need to upgrade their materials; if one Amazon review is to be believed, the front seams frayed after six months of top-down, always-exposed-to-the-sun use. My guess is they used polyester thread rather than nylon, as the former tends to break down under constant UV.