I wouldn't mind if vehicles got worn out a bit by being stored outside, which didn't happen inside, but then they'd have to rework how wear and tear works to balance it out. They might want to work on wear and tear, but there's so much else to work on, that I don't see indoor storage advantages as something likely to ever happen, unless a modder bites on it.
I did some calculations on wear and tear, and I can't say I'm overly thrilled with it. Unless there is some unspoken factor, that driving a tractor 1 km in game, should equal 100 km in real life, wear and tear just seems to happen way too fast, and the cost is linearly scaled with purchase price, so you pay much less if you buy a used vehicle than if you buy a new one, as the price you buy a used vehicle for is then the price the costs scale from. Also, you can get large gains, by buying a vehicle barebones first, and then customize it with a bigger engine and everything you want, as the customizations doesn't add to the base price maintenance costs are based from.
I'm guessing they scaled maintenance cost, so the cost per year should fit cost per year in real life, but not accounting for real life operations taking a lot more time than in game operations, that leaves you with very fast wear and tear, especially for people that try to play more realistic.
I have no idea how often farmers repaint their vehicles, but I'd wager a guess on very seldom. Repainting after every field work sounds like an outrageous expense and not realistic at all to me. But at least paint has no in game effect so I can ignore it.
Add to that, the cost of repairing scales exponentially with time, so to keep maintenance cheap, you should repair your vehicles as often as possible. If you have large fields, you could earn a lot of money, by repairing your stuff many times while working the same field. Repair costs shouldn't scale more than linearly until you hit some point of noticeable wear, so you didn't have an incentive to repair all the time.
Another thing, is driving in rocky fields.. Which only hurts paint. There's no added maintenance cost.
And to finish it of, you wear the tractor out as much sitting in it idle with the engine on, as you're doing plowing up a field with the engine using every horse in it. The only thing that matters is seconds the engine have been on. Whatever it has been used for doesn't matter at all. Same goes for implement. A cultivator takes as much damage hanging in the air behind a tractor that has its engine running, as it does actually cultivating.
All in all, I'd say the wear and tear system has much bigger issues to fix than garage effects