There are issues with quotas.
Providers have been known to blatantly lie about your bandwidth usage.
It applies arbitrary limits even when non-downloaders need burst traffic on occasion.
It does not credit you for unused bandwidth.
It tends to cost users far in excess of what a provider's actual incremental costs are for adding capacity.
If you pay for bandwidth, you should be able to use it. If a provider advertises a certain amount of bandwidth, they should be capable of, barring exceptional circumstances out of their control (it's a given that you need to plan a bit of excess usage) delivering the contracted services, basta.