Dynamic meta description?

There is a feature called

Replace the 'META DESCRIPTION' Content?

This option replaces the 'META DESCRIPTION' content with the first words of the first posting in the page to a limit defined in the option below.

What I am wondering is, is this something that is beneficial even for a new forum where I am making most of the posts myself with keyword optimized thread titles?

I had it turned on, but I noticed that picking up the first 150 characters of the post wasn't generating meta descriptions that was very keyword optimized.

Would it make sense to turn it off for now and let it pick up the thread titles as the meta description where I've titled each thread with keywords in mind?

Or alternatively, is there a way to do a combination of both where it picks up the thread title and some xx characters of the post?

Descriptions are not supposed to be keywords.... It's supposed to be a description of the topic. Short of manually writing an 'abstract' (if there were a form option/add on for this), this is the best you are going to do in an automated system. There are no options-- it's either on or off.