Yes, players are expected to commit to rolls. That’s why it’s ok to tell them the obstacle–once they’re committed, there are no weasels! It’s then up to the players in question whether or not they should blow their rewards, earn a check, or try to roll for it straight. It’s a constant battle against probability–and managing your bonus dice, earning your rewards, and planning against probability is how you get to be good at Torchbearer (and Burning Wheel, for that matter).
I think you’re looking at this upside down right now. Ideally, a PC is always playing to his traits. That’s just what’s happening in the fiction. Torchbearer’s mechanics are telling us, though, that there’s a distinction between when a Quick Witted character is merely being quick witted for the sake of quick wit, when his quick wit is giving him an advantage, and when his quick wit is somehow endangering him (and therefore earning him a check).
It’s up to the player to narrate how the latter two examples are impacting the fiction, but there’s never any reason to ever use a trait just 'cause, for the sake of the roleplay. Traits are a precious resource to be carefully spent to overcome obstacles. They aren’t “good roleplay” buttons. Traits function as an incentive for thinking in-character and being engaged with the fiction. The important point is that you only ever have to think in character and be engaged with the fiction in instances where you want those checks or bonus dice. Of course, there are other rules to help encourage players stay in-character–beliefs, goals, etc. And remember that playing to a trait even when you don’t have to will get you one step closer to earning Embodiment at the end of the session.
Consider the following example:
I want to scout the room. My GM calls for a scout test. I’m currently injured and have no checks, so I want to earn one. I also have a scout skill of 5, so my odds of success are probably pretty high, but I also really don’t want to stumble into a trap–that could kill me! I’m thinking about using a trait against myself, since I think, based on what I’ve described, this shouldn’t be too hard. I’m dedicated to rolling now, so I ask the GM what the obstacle is. They don’t tell me.
There are two issues here: one, I have no idea whether or not I should use a trait against myself. This could be ob 1, or it could be ob 4. Either way, failure could quickly lead to my death. I’m basically, as you said, guessing. While this might seem reasonable enough when adding dice, things break down when you start taking dice away. You’ll never have any idea when it’s statistically safe to use a trait against yourself–and players, many of whom are already reluctant to do this as-is, will stop doing it at all.
An even bigger issue arises when your players start to learn how to factor their own obstacles. There are rules at play here, and eventually your players are going to figure them out. I’ve GMed way too much TB not to do that when I play it now. So, ultimately, your players are going to be able to figure the obstacle out anyway. You aren’t really accomplishing anything by trying to obfuscate it.