Ripple IOU are the same as money on the Paypal account.
There are many people that uses Paypal all the day (making transactions), for weeks or months without withdraw their money on their bank account.
So the same will be with Ripple, with just some differences (from Paypal), no frozen accounts, no chargeback, no limits of transactions.
Every good gateway can become the new Paypal until there are people that trust him and his services.

Anyway, no one is forced to use Ripple, if you prefer, you can still use Bitcoin directly ... or Bitcoin on some Ripple gateway.