Monthly billing is a pain for people with non US currency accounts, as the value leaving my account is different each month depending on the exchange rate so I can't set up an automatic rule in my accounting software and have to manually reconcile it.
If there was an option to pay 12 months in advance I'd take it, and that couldn't hurt your bottom line!
Timo Ruppell commented
For small companies the manual labour overhead fo monthly billing is horrible. Please add a yearly plan.