Reason Number 1:

It is basically an excuse to give the player a ridiculously overpowered homebase.

Not only has the Sink everything you will ever need in terms of storage, crafting benches, owned bed, pure water, a mechant to buy your stuff who is also a 100% repair master ... it also has a device to teleport you there and various methods of turning useless junk into useful crafting components.

"So how is that a bad thing?" you might ask.

Well, it's bad because it effectively removes the choice from the game regarding this matter (Player housing). I don't think there are many players who complete Old World Blues and choose another homebase afterwards. The Sink is just too convenient to give up. Even if you are roleplaying you would need to make up some ridiculous reason to NOT use the Sink.

Reason Number 2:

It feels almost like a different game. Coming back to the Mojave after completing Old World Blues feels like going back into the Middle Ages after having spent some time on the Enterprise. Somehow it cheapens the basic New Vegas experience and makes it feel obsolete.

This wouldn't be such an issue if Old World Blues took place after the battle at Hoover Dam, but since you can't do it afterwards you have to come back to the Mojave.

Reason Number 3:

Cheap mechanics which make you run through the same level seemingly countless times. Goddammit how I hate having to retrieve those damn student records AGAIN and AGAIN.

Reason Number 4:

Because of the reasons given above and because I like to play the DLC in order (Dead Money, Honest Hearts, Old World Blues, Lonesome Road), I end up doing Old World Blues at Level 40+. I don't know if this is inteded but at this level the opponents are stupidly overpowered. I mean, it's not really a problem because I'm overpowered as well ... it's more annoying if anything. Those damn Nightstalkers! They don't even drop decent loot ... all they do is annoy me while I try to do the quests.

About the only things I enjoy in Old World Blues are the pretty scenery and Mobius' voice acting. SCIENCE!