In some scenarios, you need to preserve all the fuel you can get. Even if it means shutting one, keeping the cross-feed open and transferring the content of the leaking tank to the opposite engine while you still can.
In some scenarios, you may wish to keep both engines running. Even if it means with one wing already empty, transferring to that engine from the opposite good side at the expense of letting the oversupply drain out of the defective tank.
If the procedures for one of the above would be mis-applied, (that's what human elements are for), the outcome might be fatal.
Without knowledge of the E190 (myself on an ancient type), I think there will be only one fuel leak NNC. That NNC, as written, will typically need to provide one-solution-safe-for-all drill (typically fuel preservation)
After you have applied that NNC there
might be room to apply core system knowledge and sound judgement. But that is very different from the goal of the checklist's designers and people who approve such. One also
might make a very big mess of it.
Nomad2 I think the relevance to your
second experience is not with Air Transat (they did not understand the problem until too late), more with Air Asia (unauthorised reset).
See, SIM sessions have a plan and the good ones have a training objective behind them. To execute a correct NNC properly and stick with it is a desirable skill, maybe that is what the instructor was authorised to train you that day. Perhaps he needed you on one engine for the remainder of the exercises (sure, there are buttons for that at his station).