Would it be sensible to create a MySQL user for each user account of a web application?
What type of logging would you be looking at gaining? What I do in my application is an audit table that tracks application user, application command, and result (among other things). This, along with another audit table that tracks individual table changes, provides all the logging we need per user.

You could also set up a connection pool in your application to control connections per application.

At first blush, I think trying to manage so many users is a burden that outweighs any possible benefits.