At best they incur an ever present performance hit while only catching the lowest of low-hanging fruit. At worst they are constantly getting in your way with false positives (which train you to ignore any potentially legitimate alerts) and breaking everything. And the for profit ones tend toward the latter, because if you never notice it you won't believe it is actually doing anything and might not continue your subscription.
You might be thinking "that's all well and good for us computer geeks who know what's what, but what about everyone else?". Most people aren't as dumb as your ego likes to imagine them to be. They may not know the details of how their computers work but they know sketchy looking crap when they see it. And if they are that dumb then nothing will help them anyway.
No matter how you slice it, the cost/benefit pretty much never favors AV.