I lease because I like to have a vehicle that is as "Green" as possible. Since the technology changes so quickly and the components (especially batteries) are short lived and expensive, it just makes sense for me to lease. Since I can lease through my employer, I benefit from the employer running a fleet of vehicles.
For someone who is unable to lease through their employer, leasing is a way of driving an more expensive vehicle than they could afford if they were buying the vehicle.
If you put a lot of miles on a vehicle, you are just about always better off buying the vehicle. Especially if you are the kind of person who is OK with driving an older vehicle.
One of the things to be aware of: A lot of vehicles that are being sold used are Lease Returns. You really don't want a vehicle that was formerly part of a fleet like Canada Post's, Courier Companies, Utilities, Rental Companies, etc. Those vehicles aren't assigned to a specific driver, but instead are in a vehicle pool. Since the vehicle is not "my car", they tend to be driven harder and receive less maintenance than if they were in a fleet where the vehicle is leased for the driver.
If I was buying a vehicle, I'd be more inclined to purchase a new vehicle, that way I would know exactly which crazy asshole had driven my car.
With all due respect, if you are attempting to be good for the environment and reduce your carbon footprint, one of the worst things on earth you should be doing is to roll through a new vehicle every few years. Do you think the vehicle just disappears and a new one magically reappears once you've rolled into a new one?
And no...leases are not a way to make expensive vehicles more "affordable" per se, because at the end of the day your overall cash outlay will be more with a lease than with a retail purchase. With a lease what you are doing is you are trading off lower payments in exchange for lower equity and fewer ownership rights.
In addition, lease vehicles out of typical fleet pools tend to be very well maintained because they are usually from large companies that have an interest in keeping the vehicles maintained because maintenance is tracked on fleet purchases by the auto companies. Just because a vehicle is a "lease return" does not make it a vehicle to be wary - the typical lease return is usually a very good used vehicle purchase option.
You are correct to be wary of courier and rental company vehicles - but they must be declared as a "rental vehicle" or "livery vehicle" etc. upon purchase - but these vehicles are far less common.
Buying a lease return that is one year old is one of the best value purchases that you can make. They have low miles (thus couldn't have been driven too hard by that rare "crazy asshole" if you were unfortunate to get one of these vehicles), they are well maintained, its easy to tell if a vehicle has been abused (it either appears virtually new or it doesnt), the largest depreciation has been incurred, it typically still has warranty, and the vehicle is near new.
My experience in the business suggests that buying a lease return brings infinitely better value to the purchaser than buying brand new.