Yes, skirmishing smite can kill trynd while ulting...

...but from the number of bug reports i just have to ask how many trynd players are just spamming their ult when they're about to die? Skirmisher's smite is only a 4.5 second spell with a 60 second cooldown. Those junglers have to be amazingly saving their smite for the exact moment when trynd uses his ult for this to be effective. Unless of course ww has come out of the jungle and ganked you so hard that he did enough dmg to you with the smite and his ultimate alone to kill you and force you to spam your ultimate. Solution, don't ultimate when ww has you suppressed and smitten.

Most champions with a sustained ultimate can be countered with a simple silence spell from any number of champions. I've been shutdown in the middle of swain's ultimate just this way. Seems like a fair thing to be able to purchase an item that allows you to counter an invulnerability ultimate when it requires the tactical decision to save your smite for that perfect chance to counter the ultimate.

That being said, i'm sure riot has seen the dozens of posts on the subject... people don't really read other posts before posting, eh?