I'd say you'd need to start spending more time/money on solar panels if the of one really needs it to function. So far, probes seem to be getting by fine with the current panels we put on them so there isn't too much pressure to be focusing on solar panels if they're doing the job well enough.

I think someone else also mentioned this, but adding things like wipers and whatnot would also increase weight. That's weight which could be dedicated to other important parts of the craft. Yes, you'd end up collecting more power and perhaps you'd maintain your solar cells better than before if you cleaned them, but it would take research and development on something that already works well enough.

That said, solar is something that's useful down here on the ground too, so I imagine that takes some pressure off of space agencies develop the tech from the ground up.