It is reusable if it can be refuelled with hydrolox from a surface ISRU installation, while a kicker stage would lock them into a non-reusable architecture.
Also a reusable upper stage is a separate problem. I'm always disappointed when I read comments along the lines of 'lets not bother solving this problem, because I have this other favourite problem instead'. So what? That doesn't make this particular problem go away.
No, a crasher stage can do a flip and burn after separation, allowing the crasher stage to put itself back in orbit. And empty stage with only itself as a payload requires just a tiny amount of fuel to do this.
...and this would all be easier to reuse than the descent stage here.
Also a reusable upper stage is a separate problem. I'm always disappointed when I read comments along the lines of 'lets not bother solving this problem, because I have this other favourite problem instead'. So what? That doesn't make this particular problem go away.