I have my targetting bound this way, but try playing an arena using only Focus targetting, and not Arena1-3 Macros, you'll realize how much harder it actually is.
Scenario A with the change: Let's say i want to Stormbolt a shaman, Focus Fear a druid, whilst staying on my target, a Warlock. I must first shift focus, from the druid, too the shaman, Stormbolt, re-focus the druid, fear, then continue my rotation.
Scenario B now: I use my Arena 3 Stormbolt on the shaman, and my Arena 2 Fear on the druid, in perhaps, 2-3 seconds less, i have cross CC'd the team, perhaps without even knowing where the Shaman/Druid where, or who i had on focus.
I'm sure there are better examples from other classes, but i play warrior.
With how slow the GCD is a good player would not have much difficulty doing this without /cast [@arena1] macros.