In short, I don't think I present any tactical ideas that are gamist in nature. And "gamist" isn't an insult
Emdw, I don't think Dave was trying to insult you. He was more just referring to the debate, surprised you weren't on his (gamey) 'side'. For all intents and purposes, he infers that he himself is on the 'gamey' side of the fence in this debate, so he wasn't trying to insult you.
I think what we are not taking into consideration is that we have all had different experiences and with that, a variety of different dungeon masters.
Some dungeon masters are amazing, allowing you freedom of movement in their worlds, or at least as much freedom as your character can mechanically TRY to obtain through the mechanics in the game.
Other DM's are just either plain inexperienced or just... Well for me words can not really describe, but if I had my time over, I wouldn't have wasted my time with them. An example I guess might be that in a game that should start out perfectly vanilla, the DM's like "aliens abduct you at the start and mutate your DNA, at level one, you now have the stats of a balor", or after your party has tracked down the evil fighter, they successfully cast hold person on him, tie him up and gag him, but for some reason, without any magical support, the fighter magically (through plot armor), teleports away.
From what I'm getting, Dave's MAIN concern is about the ethics of RULES as opposed to RULINGS, because there are DM's out there who, quite frankly, are just plain cruddy.
I think from that perspective, most people understand and would agree with what Dave is saying, however, that really is a broader and larger debate than what the OP was asking about.
In regards to forcecage, the power of the spell really is up to interpretation. It's not as clear as it could be, there's just not enough data, so a DM's ruling is definately required. Should it be clearer? Maybe, but then again, maybe it was intended to be a little uncertain in this edition.
My only advice in the long run is to not waste your time with DM's who constantly unbalance the game. There's making good rulings and there's going too far.