Well, I don't fully know that I have to, but I just figured it was a sensible step. I know you need to pull the pump, but there shouldn't be much ATF in the housing at that point, so I guess maybe you don't need to drain it. Of course I also don't understand why the dipstick tube doesn't function as a breather tube (or why breather tubes are necessary at all, but maybe that's a whole 'nother topic).
Since you're planning on this as well, I'll tell you that I was HOPING to simply tap the existing vent hole and put my NPT 90 deg fitting right into the stock location. I read another thread here where somebody determined they had 17mm of clearance. The fitting I have in my hand, assuming it threads 100% in, is about 16.5mm, which is probably too close for my taste. I may try to find a lower profile one, but I doubt I'll have much luck.