I haven't read the whole thread, but from what I have read there seem to be a few misconceptions. From 6 April 2015 BTL landlords cannot offset against tax any mortgage interest, nor any notional 'fair wear and tear', only actual expense. So it is no longer the case that they have their mortgages paid by someone else, the tenant paying the rent (I put to one side any discussion of whether the 'tenant paying the mortgage' is right or wrong).
Before the Housing Act 1996 (which introduced the AST concept - much more limited security of tenure) the private letting market was pants: rents were regulated, and private landlords would often do little to maintain or improve properties, as they saw no reward for doing so. The supply of rented properties was also much more limited, although social landlords were more active (Right to Buy had not yet hollowed out social housing stock).
The BTL market developed because security of tenure became limited through the HA 96, and this meant that banks were more willing to lend on BTL properties. Previously, banks would lend, but would usually only do so on a heavily discounted value, e.g. 50% market value (because of the prospect of foreclosing on the security - the property - and being stuck with an unmoveable tenant, possibly paying below market rent) .
The logical thing for many people to do is to rent the house they live in (so having more flexibility if they need to move, e.g. for work) and own a BTL as well. Several people have mentioned doing this, and it makes sense. (It is also, as I understand it, common practice in Germany.)
If BTL has distorted the market, it is (perhaps) in encouraging developers to focus on building smaller properties like two bedroom flats rather than larger houses. However there are other factors contributing to that trend.