I think the issue of "grease vs. no grease/oil only" has as much to do with the design of the firearms as anything.
For something like a "classic" SIG (220, 225, 226, 228, 229, 239), grease is a really good idea and will extend the life of the firearm - as the rails are part of the alloy frame and have several layers of treatment on them that will wear down... in any event... but particularly if there is not adequate lube on there. I've had an experiment going with a 228R I bought about 3 years ago... how black can I keep the rails by making sure they are freshly greased for every range session? Answer: pretty black, and a lot blacker than by using oil.
I think this applies to some other designs of firearms as well. Beretta 92 comes to mind.
The other end of the spectrum, something like a GLOCK... little steel rails embedded in a plastic frame, running against a steel slide. That will work until the cows come home with no appreciable effect on the firearms. A drop of oil on each of the four rails is... realistically overkill... but it won't hurt it.
The HK USP's are another pistol that falls into this category.
I have been shooting a lot of HK P7 pistols. Grease would be largely pointless for these as well. Even oil should be used very sparingly on that design in my view.
But, yah... anything with a long rail, particularly one that is made of an alloy rather than steel... grease is a good idea for that in my view.