I would guess, and it is only a guess, that a large part of the choice comes from the fact that crafting jobs are actual jobs. Adding a multi-tier system to the HQ is one method of adding "content" for crafters, a way to give what we do a little more depth.

Personally, I think the idea of the system is fine. It doesn't matter if you use a +0/+1 system or a +0/+1/+2/+3 beyond the fact that it is a nice nod to crafters. Either way, the highest +HQ stats will be the most coveted by people who care about such things.

The problem isn't so much that the system is there, as it is that it seems broken. That is, we have no idea what the math behind the process is, or what affects HQ output - at this point it is mostly guess work.

Some revelation behind the mechanics would help with the situation. As will time. As someone else pointed out, as the market becomes flooded with +1, the master crafters will focus on +2. Then on +3. It is very hard to see what the game is actually like because the economy we have is still 1.) very new, 2.) not representative of what we will have after PS3's eventual release when we have an influx of players.

Honestly, the HQ system might be working properly, and we just can't see it yet because we don't know enough yet (though I tend to be in the camp that thinks it needs slight tweaks.)