have you ever tried to deal with the spaghetti "slap" code created by a developer to get a project done, that then needs further maintenance and features going forward?
MUCH simpler with conventions, whereas otherwise you're dealing with B.S. quality code, trying to find what function does what/where, and figure out why the hell the previous person copy/pasted his code into 20 different files instead of just creating a single include or function.
Conventions are good to have. I agree frameworks will slow you down if using for just one project with no prior experience, that is), but conventions will save your ass past the initial completion date of a project.