A poll might be a useful (if limited) workaround if the person posting the entry actively wanted to make a private commenting feature available on a particular entry.

However, as I understand it, the original issue concerns a feature as universal as commenting or private messaging - something that can reasonably be associated with an entry or journal without much effort on the part of the poster. In addition to the limitations you've noted, adding a poll to an entry is not something that happens automatically, so it requires recognition on the part of the poster that the option might be useful to their readers and effort to create the poll and include it in their entry.