If I do not ever plan to buy night vision, is there any reason to buy the M2 over the ML2? It's only $30 more...

From a pragmatic standpoint, no. But if you're like a number of us, we like to have as close to military configuration as possible.  The extra $30 isn't so much for that. Also if you decide to sell it, you may have more takers. And who knows, just one day you may get a hold of NVD when the prices drop to $99! (Just dreaming)

Lastly I believe (and I'm not sure of this), if the M2 switch happens to be inadvertently knocked off the "off" position a click or two, the battery drain would be less than on a ML2 - as miniscule as it already is.

There is no such thing as an "off" switch or "off" position on an M2.  It is always on.

Always ON? Why would they do such a thing? So does the ML2 have an OFF switch then?

I like the suggestion about it being more valuable/desirable for resale, that may have settled the choice for me.

The reason there is no off switch on the M2 is because it would be useless. Batteries lose a certian amount of power by just sitting there unused. The M2, on the lowest setting, requires less power from the battery than it loses by just sitting there. So, just leave it on the lowest setting. BTW, the lowest setting is only visable while wearing NVDs.