Only bother with the 5v5, as the others are a waste of time if you're just interested in points.

Basically, you get 100% of the points that your rating would give you, if you were in 5's.
In 3's, you get 60% of the points that the same rating as a 5's would get you.
And 40% in 2's.
That's a rough approximation, but if you're not on a highly ranked team, 5's will be all your points.

You don't actually have to enter the arena.
You can queue your 10 games a week.
And then not bother to enter, which is a default loss.
But you're not wasting your time, and are earning the points.