All a written contract really does is state the percentages, etc.. everything you already know. And sometimes they have a 'either party can dissolve this agreement with x amount of time' clause. Some don't.
If you know beforehand how the agent does business, ie, sends you an update monthly, weekly or never, lets you know who's looking, who's rejected, if you have to reimburse for fees, etc, then there should be no problem with a verbal agreement. Just make sure you ask all the right questions before agreeing to be represented by them.