You are not looking at the whole picture though. I've never seen someone with a pistol down a Grey in a single shot (albeit I have to admit I hardly ever used the pistols) while I've seen it plenty of times with a riffle. It been a long while, but I think that riffles have more ammo in their clips too. Another point to consider is that if it take you more shots to down an ennemy, your odds to hit increase by firing more shots, but so does your chances of having extra casualties due to alien reaction shots in your turn. Does each weapon also have a reaction time too? It been too long since I last played so I don't remember well, but I think that it was also a factor for me since I rely a lot on reaction shots in the ennemy turn to catch those pesky aliens off guard. Having a more powerfull gun always help to be lucky and down them in one shot, saving the life of many men in the process. That's also where autocannons and the big gun come into play too. That, and the fact that they have specialised ammo like the inciendiary clip, which is a killer against the Night Ones (who turn civilians into zombies) and can also serve as a light source in night missions.
The only real advantage pistols have over riffles is that thrown item accuracy is way better with a one hand weapon than a two hand weapon like a riffle. Two hand weapons like the riffle also lose accuracy when the squaddie is holding something in his off hand, while pistols don't have such problem.
But even then, I too always look to change those beginning weapons ASAP. It simply that I tend to overlook laser weapons completly and research plasma weapons right away instead. You really don't need better weapons until about month 3 in the game if you know what you're doing.
From what you are saying Japofran, you seem to rely on grenades
way more than I ever did. I prefer to kill the aliens with hand weapons instead, reducing the chance of destroying alien equipment that I then don't need to manufacture myself. I hardly ever need to make my own plasma clips, for example...