It depends on your tastes. If youre into HK's, Sigs, and Glocks -- yeah, youll have 13.5 LH barrels. Possibly.
Im a host collector, so let me run down the 1/2 x 28 9mm stuff I have that I can remember off the top of my head.
Glock 17 (LW barrel)
Beretta 92
STI Spartan (ADCO job)
CZ 75c (ADCO)
CZ SP01 Phantom (ADCO)
CZ 75B (ADCO)
CZ P01 (ADCO)
CZ P07 TB
Walther PPX
Walther PPQ Navy
Sig P226 Tacops (see note below)
Browning BHP (ADCO)
Springfield XD Tactical (EFK barrel)
M&P9
Nagant 7.62 revolver (ADCO)
Maybe a few others, but you get the idea. Alot of aftermarket barrels come in RH threads, and for good reason. Also, 1/2-28 boosters allow you to use your takedown pistol cans on 22 hosts. I do that from time to time.
For 13.5 LH guns, Ive got a few western European ones.
Sig P226 Tacops (I have a 1/2 28 barrel for a P226 knocking around somewhere, but I dont
use it as the only 226 I have right now has a factory 13.5 LH barrel.)
Sig P229 Elite
Sig P239
Sig SP2022 TB
G19 (Gen 2, old as shit, has a factory barrel now)
USP 9 SD
VP 9 (RCM barrel, could have gotten it in 1/2 28 though)
Ive got a couple others I cant recall right now, but as you can see, my taste just happens to trend towards guns where the 1/2-28 barrels are more common or readily available. Its all a preference matter, but neither thread pattern is "useless". Also, 13.5 LH doesnt resist unthreading in any appreciable, noticable way. Im not an expert on anything at all, but Ive shot alot of hosts with alot of cans, and they all need to be checked for tightness after every mag.
The same goes for .45. I prefer .578x28 over 16mm LH threads. I think I have a HK45CT and a factory G21 barrel in LH thread and thats it.
I think I would rather have a factory barrel in most cases, but if I cant get one or I intend to shoot it ALOT, i get RH threads.