A friend ordered an Aimpoint Comp ML2 for me and mistakenly specified the Night Vision model. I don't have NVG now nor do I ever expect to.

Aside from the extra $40, most of which will be eaten up in return shipping, is there any reason I shouldn't just live with it?

I ordered a ML2 from a dealer and they're still en route from Sweden (and have been for a LONG time).  So, I'm getting a M2 instead he's eating the difference.  You lose a few day settings, but I don't think it will be a problem at all.  Plus, you get the military-issue cool factor [:D]  Keep the M2 and enjoy!

Correct me if I'm wrong but isn't it the Comp M2 that's designed for use with night vision and the Comp ML2 is not designed for use night vision?  

Yes, M2 is for night vision, ML2 is not.

The M2's first 4 settings are for night, and believe it or not I have caught a glimpse of setting 1 while aiming the sight directly into a pillow at point blank range.

For sake of arguing, seetings 1 and 2 are virtually invisible to your naked eye, but 3-10 are quite visible.

Yeah, I ordered an ML2 and got the M2.
Probably not worth the hassle of returning it.

The battery on the M2 will last for 10 years on the lowest setting, making an off setting a purely academic issue. The remaining 6 settings are all you'll ever practically need (what would you really need w/ 10 brightness settings anyway?). I live in WA state, where the whether is all over the place, & 6 day settings covers it. I've only used settings 5-7. Plus, the option is always there for NVD's, should you ever get the craving.

I have the M2, but I don't have no stinkin' regrets. [:)]

I have a new aimpoint m2 works great at brightest settings (8 thru 10) with new batteries. ( 1 thru 4 ) for nv. 5,6 & 7 very dim, 5 & 6 even in total darkness. Anyone know if this is common for 5,6 & 7

Read my above post...
I never go past 7 due to it being WAY too bright above that setting. I think you should check your batteries...they may be new to you, but that doesn't mean they are new.