Then they can have some fun results with how you deal with those clients. Like they've said for Skyrim, you can put someone out of business. How about finding a creative way of doing that for clients who stiff you? Tranquilize a bear and put it in their shop. Steal stuff from their home until they agree to pay. Take them to court. Mug them when they're heading for the bank.
To make these possibilities possible however, they're going to have to make the client pay you directly, and then you give some percentage of your pay to the guild. That feels more natural anyway, and is likely what the guild would want because you aren't going to get every quest from the guildhall. What if a merchant asks you while you're in the store:
You find me in desperate straits, my dear Redguard. I have received word from some Gothway Garden thugs that they intend to ransack my house tonight. I don't suppose you'd be available to do a little creative facial rearrangement on these fellows?
That's a Daggerfall quest, and something the Fighter's Guild would totally do. If you're already in the Fighter's Guild, shouldn't you have to pay your due? If you weren't in the Fighter's Guild, wouldn't you lose reputation with the FG for being a freelancer taking away work from them? Avoiding paying the guild dues if you're in the guild should be a crime like tax evasion is; You're only in trouble if someone finds out.
Then for weirdness of payment, take Fallout 3's Harkness quest. (Spoilers)
Spoiler
What the heck? If you reveal to me a startling fact about my life, I'm not handing you my tv or something, that's straight goofy. Adventuring quests aren't the kinds of things that you can make a table of standard costs for, you should haggle some and they should really try to lowball you sometimes. I don't mean they should do this to the point that it isn't fun, but I'm tired of every client being upright and dependable. If you tell him he's a robot, he gives you an awesome gun.
Anything you have to add about dealing with clients in Skyrim, say that too.