I'm going to assume you don't mod very much. Basically, there's two main issues.

1) Modding can quickly become very expensive. If Ordinator costs £1.50, Frostfall costs £2.00, Legacy of the Dragonborn costs £5.00, CACO costs £1.50... A large mod list becomes expensive to maintain. People either won't bother to mod (and thus the scene will die) or new mods will struggle to get noticed as people will have already blown their modding budget on the big names.

2) Modding is voluntary work. You don't volunteer at a soup kitchen, then after 3 years demand to be compensated with a salary. You get little perks (in the case of modding, donations), but you went into the work knowing it was going to be unpaid. It's not like the modding community are forcing people to mod against their will; the modders are doing this voluntarily.

It sounds like a good idea in principle, but paid for mods is a good way to kill a modding community.

The current method Bethesda are thinking about adding isn't as bad as "paid for mods", but it has the very real possibility of devolving into it. People are scared Skyrim modding is going to take a hit, or worried Bethesda are going to try and nickel-and-dime every mod they can.

At the moment, we have to wait and see.