For low-density branches ,such as chemical, how significant are platoon leader positions?

It don't matter what branch you are in LT. The purpose behind the 2LT is to learn what it is like and how it works down in the weeds so to speak about your job. That way when you go up there in the ranks you know what you are talking about when you make those command decisions.

While differing to officers in officer matters, I would recommend any leadership time a leader beyond corporal can get. These are lean times in manner of promotions and retention. You need to build your resume as a leader to keep your career going.

If its an official MTOE slot then by all mean go after it because its important in some aspect. Obviously someone thought it was important to assign it to officers so it has to be important and someone has to do it.