I do so for two main reasons:
1) Yes, it can be a money loser. In some tanks prem ammo shells cost 10-20 times the cost of regular ammo (others just 4-5 times the cost). I am not cheap, but I do keep a mental watch on my game "finances". Yeah there are times when I do need to just learn to fire prem to stay alive and even help win the battle more.
2) It's a challenge. I HATE when I see people (OK, streamers) load 100 prem ammo (plus a few HE shells) and just hop into battle Skill does this usually when he's over his 95% on marks and is heading to 100%, and I really cannot say I blame him because that way, if nothing else, he can just focus on gameplay and tactics rather than switching ammo types. But then there are others. Nick_Shox. Not a huge fan but I do watch him on occasion; I honestly find him to be kinda boring. I watched him the other day. Grabbed his T95/Chieftain with 100% gold loadout. And then proceeded to rule every battle. But why?? He is a great player. I've watched his streams and he's got situational awareness and a really great feel for the game; I mean a GREAT feel for the game. So this time he just puts on his big boy gloves and goes to town. In the several battles I watched I think he penned almost every shot, including angled big ass tier 10 superheavies.
Where is the challenge? Why bother? And why bother doing so on a stream? I'm sure he had a "valid" reason (I think his Twitch title was "Testing something or other"). So try it for 2-3 battles and then go back to being a normal player.
Yeah I'm singling him out because it was obvious and it's fresh in my mind. Others do this too though. And yeah it's a game so "Do what you want" but there is supposed to be a challenge aspect to it. If you go all-in, where's the challenge?
I know — I'm preaching to the choir and everyone knows that everyone else spams gold. But I do not. And I don't get why some do. It's just not fun, at least to me.
Oh, and as for my initial question … thoughts on my own mindset when I should switch to prem vs regular ammo in-battle? I've only been playing since 2011 (seriously), but still continuing to learn lots about nuances of this very complex game.