This applies less to level design and more to gameart in general but, I think its definitely worth, just not in the sense that you mean.
I find modelling fun, and its personally satisfying for me to be able to take something in my head and bring it out so that others can see. Money or building a portfolio isn't a motivating factor for how much work I do on a personal daily basis, its simply about how much fun I'm having.
Now, when I get a job at a company doing this, things will change and I'll likely be working just as much out of love as for the paycheck at the end of the week, but until then . . .

Anyway, right now in the freelancing community people who will actually pay you for your work are generally a minority, and a small one. So if you just want the money, you'd be better off finding another field because checks will often be small and far between.
This doesn't mean that you have to give away your free time and skill to some project run by a person who has no idea what they're doing to validate your love for art and games. Just practice everyday, keep looking for work or reliable looking projects to join and hop on. And make sure you have a part-time job in the background keeping the lights on and food in the fridge somewhere in the background. ^^