how is it strictly worse? both option let you cast sd whenever you want. But if your wand gets dispelled eg in first qc, your sd is also gone, while with the tome you still have it.
Also switching is free with tome
I would still prefer the wand, as it preserves spell points after the second or third use.
There's no switching if you keep Seeking Dispel as your 3rd option all game, as per Rinc's thought experiment.
With the Tome, you use up the SD every time you cast it; with the Wand, you never run out of SD as long as your opponent doesn't Dissolve your Wand.
You're right, though, about losing your SD if/when your Wand gets Dissolved, but only if you never cast your SD. As long as you get 1 use out of your Wand (compared to getting 1 use out of your Tome), then the Wand is strictly better in a situation where you always choose SD as your 3rd spell option - even if it gets Dissolved.
(OK, there's another situation where a Wand isn't better: if your non-Wizard opponent uses [mwcard=FWI09]Steal Equipment[/mwcard], they'll get to keep your Mage Wand of Seeking Dispel, whereas they wouldn't get to keep your Mordok's Tome.)