I lived in London and saw that first hand - also houses bought to rent out, but with no rush to bringing to market - they get to the market eventually, but this can take months.

I now live in a small seaside town and one of the issues here is people buying 'cheap' houses to own as a holiday house. They might be here every weekend, or maybe just a few weeks a year. This is taking away a house from a 'local' - someone who would either buy or rent to have as their main residence. To this end I'd rather see them on AirBnB than just empty - at least that way people are enjoying the house/location and spending money in the area. (Though I'd obviously rather see someone who lives and works here able to afford the houses in the area).