I'm on XBOX 360 Vanilla and I have been playing mostly Melee based character.
I got to the Symbols of Office Quest in SI and didn't read ahead for spoilers. I got to the Crystal Lake battle and had been trolling through the area in my 100% chameleon outfit to level up my marksman and destruction skills. I got to face the doppelganger, he used a bound dagger, I killed him, he didn't drop Shadowrend.

I didn't realize I missed it until I read the guide later and went back to see if I could collect. No dice.

I saved post-fight.

Am I missing much by not having Shadowrend?

I enjoyed using it, but if you're not a fan of 2-handed swords (or axes, depending on your skills) it might not be worth it. Although, if you enjoy collecting rare items, it's gone forever unless you reload and force him/her to use it.

Not really, though that's just because I personally favour one-handed longswords. Shadowrend looks really cool in my trophies room, but I've never really used it much because it's two-handed.

It kind of depends on what level you were when you would have gotten it, as it is leveled. It's good if you're approximately level 25+ when you get it, if I remember correctly, but before that not so much. It's nice, but I personally wouldn't go to extreme measures to make sure I got it if I didn't get it the first time.