Well, no, it isn't (particularly fragile) I'd say. Or well, it might depend on how you treat your watch perhaps, but I charge mine daily using Apple's supplied puck, and the back is
a little fuzzy after nearly three years, primarily in the very center of the back*, where it doesn't matter. The area around the center where the LEDs and photo diodes are located does have visible microscratches, but they're so fine and so sparse that it can't possibly affect the functionality of the pulse sensor to any measureable degree, and that's after three years (in a few weeks.)
I just wouldn't worry about it. If you're a little OCD about scratches just keep your watch laying on its side when you're not wearing it, to keep the back from rubbing against any hard surfaces. Still, I don't think it's a big deal. Be happy with your choice and the nice deal you made. The plastic back is also lighter than the ceramic one, so if weight is a big deal to you (like if you're a runner for example), you'll save a few grams here. Think positively!
*Which is plastic actually - technically composite, since it's a multi-injection molded piece with fresnel lenses set into it, but "composite" is really just a fancier way of saying plastic without having to use that word.