As with so many issues, it all comes down to your by-laws.
Our building has a by-law that specifically forbids letting car spaces to outsiders – but we know it goes on.
It’s not an issue because it hasn’t caused a major problem (so far) but it does leave us open to abuse if, for instance, the person with the key allows their mates to park illegally when they are all going out somewhere together.
The fundamental issue is the security of the building and how badly it is compromised by outsiders having keys. A person who is not an official tenant does not have to abide by any by-laws, including restrictions on parking in visitor spaces, for instance.
If you don’t have a by-law, you are wide open to anyone and everyone coming into your car park and possibly your common areas and you have absolutely no control over their behaviour.
If you do have a by-law, you don’t have to use it until there is a problem. Then you dust it off and let it do its job by sending a notice to comply to the offending car space lessor.
Here is an example of a couple of clauses in a fairly typical by-law:
No part of any car space may be used by persons who are not an owner or tenant of that lot or a visitor of a resident or owner of a Lot.
You must not
(a) grant any lease or licence; or(b) sell, trade or loan any part of any Lot intended for use as a car space other than to an owner, tenant or resident.
Some buildings have even tighter restrictions that say the car space can only be used by the resident or their visitors, and not the owner who is not a resident. This is to prevent owners “double-dipping” by holding on to a key so that both they and their tenants use common property facilities.