1. is it bad for individual "professionals" to invest capital on a small scale to provide services that people want at a greater or lesser price? Be it cars or houses.
2. If it is bad, why, and who for? Is it bad for consumers or bad for existing businesses?
Furthermore, unit owners in these buildings paid for their homes with an expectation, based on the law, that their building would not be used in this manner. By violating the law in this way, the AirBnB landlords are affecting the value of these unit-owners' property.
On one hand, traditional regulation keeps small or part-time players out of the market, because regulatory overhead is too high. The relative burden of regulation on large companies is much less, because they can amortize the cost across more business.
On the other hand, this only works if these smaller players get relief from regulations -- which means that they are operating outside the laws and regulations. For instance, turning an apartment building into a hotel via Airbnb sidesteps rules around zoning, tax collection, housing policy, etc. You could decide that these regulations are just annoying red tape, but they were put in place for legitimate purposes in the past and it seems like we ought to at least consider the impact before we throw the baby out with the bath water.
A third argument is that peer to peer marketplaces are changing the balance of power between producer and consumer. As a consumer, this sounds great -- but the person at the other end of the line is now an individual, not a giant corporation. When I have a bad cab ride, I go on Facebook and bitch about it, and life goes on. When I have a bad Uber ride, I give the driver one star and they may very well get fired -- and Uber will just replace them with one of the teeming masses hoping to make it rich driving strangers around for a living.
I suppose it discourages some, but it's not like there aren't small or part-time players in property rental. There are a lot of them! Small-time landlords who own a few rental units are everywhere; I know two such people in my circle of friends who do that. You can either do it as a part-time job, or hire a management company for a fee to oversee the actual management. The difference is that they do it legally. They own apartments in buildings where it's legal to rent out the units, and they rent them out legally.