My previous coach was a 36' DP and didn't need a tag. A must have for my new coach was a tag axle. I love the addition of the tag and all the benefits. The extra carrying capacity was low on the list for me. I primarily wanted it for the straight line stability, extra brakes, ride enhancement and it looks cool.
After purchase, I found some extra benefits. One, it turns sharper than my 36' and it gave me a place to rotate my front tires to. So even though you're buying two more tires, I've found that I can get extra miles out of my tires by being able to rotate the fronts with the tag. On my previous coach, if you had any wear issues, or just the normal front end wear, there wasn't much you could do about it.