Towers should focus the person doing the most damage

I've always thought towers were pretty useless. Specifically if your tanky enough to just sit in it.

We've all been there (Dyrus especially) your just farming under tower and BOOM a towerdive, the tank just soaks up all the damage and your at the mercy of an assassin.

Shouldn't the assassin be the priority? We teach players not to focus the tank so why do towers?