I did it just to be different, for various reasons I don't any longer. Biggest two reasons for me were mine was a Texan and it is a true POS, and it is a one shot hunting rifle. You have to know the expected temperature range of the day of the hunt, and fill to a pressure that will work for one shot the entire time of your hunt, always being careful to not let sun on the bottle if the temp is rising during the day. The pressure change with temp change very easily takes you out of the range you can trust the accuracy on a texan, and if it gets warm and you let sun hit the bottle, hello massive valve lock even if you filled to the low end of pressure for one shot accuracy. I discovered that last bit attempting a less than 50 yard shot on a coyote in late afternoon while deer hunting after temp had gone up 30 deg F from morning hunt. I had filled appropriately for that temp rise and should have been ok, but the dappled sunlight added enough heat to the bottle to valve lock me, shot blooped and likely hit the ground less than 20 yards from muzzle. Pressure on guage after that shot showed I was still in valve lock territory.
Then you have the extreme length of pcp's for hunting big game, sucks in thick woods and most blinds. If I want quiet/light/short range I'll take my AR pistol suppressed, skip the quiet and I'll take my favorite handgun in 375 supermag, or if I feel like being extremely careful, I'll take my old S&W 41 mag, but always afraid of putting a mark on it since it is worth too much money these days to take into the woods, one scratch would likely be a several hundred dollar drop in value. I still take it once in a blue moon.