Whilst I think it is probably true that for most new players at the average site, a pistol should be fairly low on the list of purchase priorities, there are some sites where it is pretty much a necessity, and one or two sites where a pistol is far more useful than a rifle, and that includes my local Trojan site at Weir Mill.
Weir Mill has eight floors (seven of them in use generally), there are two stairwells, both with two flights of stairs between floors, with a 180 degree angle turn in between flights. If you include the entrances to each floor in the equation and the landings, that means there are about 84 right angle choke points on the site, all of which are places where you can only use a pistol (no frags of any kind on the stairs). The rest of the site on the floors is semi-auto only, but you can use grenades there. So you actually do better dual-wielding pistols and taking no rifle at all, than you do with just a rifle, and I am in fact considering doing that this Wednesday. I've also considered dual wielding MP5Ks on single point slings, with dual pistols too, which might be kind of fun since it'd be a light (ish) load out and would offer the best of both worlds. It's also pretty much a necessity to have a knife at that site too, since whilst you can use your hand to foo a knife kill, you do sometimes need to throw your knife on the stairs. I did once nail an entire team on those stairs with my knife; they were stacked up ready to go through a door lol.
Weir Mill is pretty much an exception to the general rule I know, and presents some unusual challenges, and it is a site where radios can be quite a big factor too, but it is not the only site where a pistol is a very good thing to have.