When I was in the role of gunner in our three man machinegun crew, I carried a pistol. I was also issued one while in Somalia on account of being temporarily assigned to a vehicle convoy security detachment. (which was fun!)
I had a rifle or a machine gun in both settings, but I was darn glad to have the pistol. I never shot it in combat, but it was drawn several times either because someone was close to getting shot, someone was being covered while being searched, or simply to back an individual up.
In all of those cases, a rifle would have worked, but not as well. I don't want to make myself out to be some super tough guy that has seen the elephant more times than Hannibal, because I am definitively NOT some high speed low drag guy, but at the end of the day, in those particular situations, the pistol was the better tool. In all of those situations, I was close enough that I was able to move around somewhat more easily what with all the crap we had to wear. Additionally, I was able to retain more control of the weapon, and at the ranges that I was at in these cases, ballistics were sort of a non issue. That round would have done whatever I wanted it to do, basically.
Would I want to fight with a pistol? No! It was purely a defensive and back up to my main weapons. It served a particular role very well, but in the grand scheme of things it was not in any way a primary weapon. This statement in no way makes it "obsolete", by the way. The weight and space issues involved in carrying a pistol with a couple of extra mags are negligible when compared to the advantages of having a back up firearm.
The biggest problem, in my opinion, is that pistol skills are not (or at least were not) taught particularly well. I had a familiarization plus annual quals, and that was it. Not such a big deal for me as compared to other guys since I grew up shooting handguns, but I personally felt underprepared to use a pistol in combat. I did it, and I am certain I made every mistake possible in doing so.
As an aside, I liked then and like now the M9. Obviously a whole lot of people don't, but I always did. Easy to maintain in the field, accurate enough, reliable. My issued M9 was bashed around a lot over the time that I had it, and if I could have bought it when I got out, I would have because it always worked. Always. The gun is perfectly fine, it's that crappy ball ammo that I have a problem with.
Also:
What we carry would have killed a Roman soldier
Assuredly, it would not have. I would agree that many Marines and soldiers carry ancillary stuff that is rarely, if ever, required on patrols, and I am the first person to say that a big knife is kind of a dumb thing to carry on patrols because of the extra weight (with no particular pay off in utility over something not as big), but for the most part a human being in reasonable physical condition can easily haul the weight you are talking about. I am not saying it is fun, but with modern load bearing equipment it isn't exactly torture either. Then again, the 7th Marines motto is "Prepare to March", so maybe I am just biased.
