These filters are not very useful to keyboard/mouse users unless you are a Tab user.

I have a bigger question: Why are summons and companions target-able in the first place? I could see this being relevant during PvP, but other than that we shouldn't be even able to select another player's summon or companion.

As a tank the few seconds you waste selecting the wrong target can mean your friend obtains too much hate and dies.
As a healer the few seconds wasted trying to hit the right party member can mean certain death for your friend.
As a damage dealer the few seconds wasted can mean you lose a substantial amount of dps time.