So, this assumes that you're able to successfully planar bind a Nightmare. It isn't too hard, as long as you do your homework. I noticed it as I was perusing minions for my Malconvoker.
Nightmares have 6HD, so you can planar bind them at level 10 as a malconvoker, or level 9 as a straight wizard.
They have Astral Projection as a Su ability. Nothing in it mentions it being used as self-only, and they're smart enough that you can tell them to project your friends. They have a CL of 20 when using it.
Since it's a Su ability, it doesn't have material components. Whee, you just saved 1,000 gp or more! Oh, also, since it's normally a 9th level spell, you've gotten access to it 7 or 8 levels too soon. Woo!
Okay, so what's the deal with Astral Projection? It's the perfect "extra guy" spell. When you project astrally, your normal body becomes suspended. You can project FROM the astral to any plane, creating a new body on the new plane. That body has your equipment (but it's just a copy). If you're killed on this plane, you just pop back to your original body.
Even worse, the mechanics for projecting from the astral back to the PM are unclear. Do you just get a normal extra guy? Many people think so, but it's at best an AYDM thing.
So, yeah, astral projection is cool. But natch, it oughta be cool: it's a 9th-level spell. It's just that it happens to be usable at will (!) by a 6 HD outsider. Time to start figuring out how lesser planar binding works.