...is that some days, you don't have any ideas that would produce a piece beyond your average quality. Thus, by writing that day, your average quality of writing goes down, not up.
What's the alternative? It's to write more selectively, and publish only your best and most worthy work. It's the approach Paul Graham takes with his essays, for instance. You get around a dozen a year, and they're all amazing.
However, I think the greatest danger in writing is in not doing writing enough. Being able to do a decent piece when you're off or completely uninspired has value, and you'll bring your "skill at writing when uninspired" up (and such a skill certainly exists).
But how about your readership?
That's tricky. You always want to write good things for your readers, so that their lives or enriched. It's tough when the gears are grinding and the output isn't that great. If you find yourself in that scenario and you're uninspired, I'd give you one piece of advice -- keep it short.
A short, intelligent argument, quote, or interesting fact keeps the train of writing going along the rails. And I feel like it's hard to write a bad very short piece. You make a succinct thought with an interesting little bit to back it up. It'll at least be somewhat thought-provoking.
Yet, still, it can be torturous to write during those uninspired moments. But if you stop writing, it can be very hard to get going again with the consistency needed to really build your skill, put great thinking and arguments together, and connect with a readership. And all good writers have down times -- so you'd best learn how to write something OK during those periods if you want to write.