It seems absurd to me that while Clark and others are making money "melting" the sharp edges off 1911s, Sig is engaged in adding a sharp edged scope mount to their dustcovers.
I believe SIG's main purpose for the rail was in response to requirements of government contracts more than anything else, but I'm sure the gaining popularity of them was also a consideration. If you look at almost all the major makers offerings these days, the rail is available on most models.
As far as holsters go these days, I order mine made for the rail, that way I'm covered, so its no big deal. The non railed guns work fine in the larger holsters, the railed guns dont fit, at least in most model specific molded holsters, made for non rail guns.
The rails on all my pistols do no extend beyond the edge of the widest part of a normal frame, other than at the dust cover, so they really are not "bigger" except for your holster.
I didnt really like the rails at first, but they do offer that ability to light or laser your gun at will. I understand what your saying about your scenario Handy, but that is only one small part of it. The gun is a lot more versatile to everyone with the rail there. You can still use your hand held, even with the light on the gun, depending on the situation and when you need your off hand for other things, you still have the light available, so there is really no wrong answer, unless of course, the rail itself is the wrong answer for you. In that case, dont buy one.