From a technical standpoint, I disagree. Customers tend to get in the way of good code, they have everchanging requirements, set deadlines and priorities that more or less force you to be messy. Experimenting with techniques, redoing things, etc... is hard when you have a customer expecting this or that feature to be implemented yesterday, and yet, it is important if you want to progress.

Working with a customer will make you a better businessman, not really a better programmer.

Doing a project with customers is extremely valuable if you intend to start a business, but if you hate that and just want to be a good coder and make a living out of it, there is a solution: find a good boss/manager. He will take care about the business stuff and allow you to put all the skills you got doing toy projects to good use.

Otherwise, if you want to do something that looks more like a real-world project without the hassle of dealing with customers, make software that you really use, and share it. A good candidate would be a video game that you enjoy playing, or some tool for a hobby of yours, or a utility. That way, no one will stop you from experimenting, but you still need to think about usability.