None of those use cases concern me - the ones that do are arrow-shooting and object-throwing. Maybe if it was completely transparent, but it smoothly became more visible when the frobber is active? That would be cool. I would certainly at least try it if it was well done and subtle enough not obstruct the view or otherwise deteriorate the gaming experience. I'd await the teams feedback, and if they give the all-clear, please go ahead and create the crosshair. Please note that I am not speaking for the team, but I am under the impression that they do not mind if others create high quality features which could be added to the mod. The only logical and concrete reason not to have it is the lack of a motivated/talented person interested in investing their personal time in the creation of the said crosshair in such a level of quality it would be accepted in the mod.

Many preferential things are subjective things.

'It's not in the spirit of dark mod/thief' doesn't count, because what constitutes the spirit of dark mod/thief is a subjective thing, and with the option, people could enable or disable the crosshairs as per their individual conceptions of the spirit.

Is there any good reason not to have it as an option (disabled by default)? And by 'good' I mean logical and concrete.