This is the way SHUN used on sukhoi forum for some models.
OK, I'm not original
The problem is many models are constantly updated, so you need to make the second lod for them every time.
Don't see any problems, except lazy of model author.
How does framerate change?
I don't know, but I'm pretty sure, that game engine will work MUCH smoothly with 2 LODs. Which has 5000 triangles
less, than with model which has 5000 triangles
more.
I doubt it's even worth the effort in case of Marder.
Then don't do it. It's easy.
You'll win maybe +1 fps at best.
May be YES. May be NO. But as my point 1 fps cost to spend a bit efforts. We'are not forced and it's a hobby. No need to be in rush.
In case of 38(t) or Tiger it could be good though.
At least, it won't be bad.