Against a champ with silence, sure, it hurts him a lot, but the opponent should be aware of the CD on it, and Morde should be smart enough to invest in tenacity if that's the matchup.
Still, early game, depending on how Soraka vs. Morde decide to position, Morde can still potentially Siphon safely the 3 minion wave, queue up Q (and maybe W) before entering Starfall/Silence range, then slap the minion wave and proc it even if silenced. If it is 3 minions, the last proc off Q will likely still hit a Soraka who stays in range, and Morde has probably only taken damage against his shield - or regardless, spellvamp. As it evolves, either champion can go back and forth with counter items, or the superior strategy may simply be a lane swap, depending on how windmill fights are going.
It's a cute and annoying little magic slap fight for both sides, but Morde is still going to clear minon waves with ease, and is just one of those champions who should at the very least force a stalemate if he's not a complete noob.
Odyn's is certainly very dangerous in a committed fight where your MR has been shredded. No doubt. Great item.
Talon's silence is too short to matter, particularly with tenacity. The question for any champion is whether or not they have enough health & defense to survive the inevitable burst. A smart Morde with Sweeper will already have Q queue'd up to beat Talon's face in on the teleport, thereby foiling the invisibility as he transitions into his ultimate.