And then there were ten

I was reading the most recent primary comment thread at Kos and someone suggested ignoring the debates till some winnowing occurs, because ten is too many to keep track of.
It does seem as though only declared candidates should be invited to the debates. Are they all declared now?
I know the declaration is a formality with fundraising ramifications, but why not use it as a way of limiting the size of early debates? If you’re in, get in. If you’re not in yet, why should you be up on the stage?