It's nothing to do with tarring all landlords, surely? 
It's just that for a tenant, it will be much nicer to feel secure. I think there are lots of landlords who wouldn't imagine what they were doing was anything like a 'revenge eviction' - but there's no real pressure on them to consider letting tenants stay, either. The obvious example is when you have a baby. At the moment, there's nothing to stop your landlord, perfectly politely and calmly, declaring he or she prefers not to rent to a person with a child and serving a section 21. Or, if you tell the landlord the boiler is broken (and still broken, despite their attempts to repair it), they can serve a section 21. It could be they prefer to do repairs with an empty house, or it could be they want to get someone else in.
I've had both of those situations. I don't for a minute think either landlord thought they were doing anything unreasonable or unfair, and both of them were perfectly polite. As the law stands, they were quite entitled to do this.
The difficulty is that, since you don't get your deposit back before you move out, it can be very precarious for people to move, especially if they end up moving multiple times in a short period.
This law isn't about saying all landlords are big meanies. It's about providing a bit of security.