Move Speed can be one of the most important Stats.
However, it as a diminishing return on it... or a Move Speed Cap (not sure what it is, or if they dropped that actually, but outside of the Fountain buffing you on your way out there at least used to be a set cap on it).
Stacking Wind with Mobi Boots gives insane map presences... stack a Sivir Ult (or any champ with a MS Steroid, like Blitzcrank) and you can close on prey like a pack of Raptors in Jurassic Park (the one with Jeff Goldblum, it's Chaos Theory or NOTHING!).
But what it does most is allow for a change in Itemization. By this I mean, if you get Wind Dragon... you can sort of toss the Mobi Boots (unless the Enemy get a Wind Dragon, evening the field). So now Champions that take Mobi can swap them out once they secure a Wind Dragon for something else... AS, MP, AR, MR, CDR.
So I like that part of it, and it's pretty strong in that regard. I mean, you aren't going to swap out a Damage Item because you got the AD/AP Dragon. You aren't going to toss a Sustain Item because you got the "Every X Seconds, HP/MP Regen" Dragon. You won't drop your Jungle Item because you got the "X Damage to Large Monsters and Turrets" Dragon... the only Dragon that actually leads to a more flexible build, IS the Wind Dragon. Even if it really only effects Boot Options. I mean maybe you by-pass Dead Man's Plate, but that is OOC MS PLUS Burst, so it likely stays in Builds it currently is in.
Now, I do like your idea... Keep the CURRENT OOC Move Speed Buff and Half it upon taking or receiving Damage (combat) or being targeted by an Ability (Banshee's, Cleanse, QSS... still lose the MS).
Just clarifying the real 'hidden strength' behind Wind Drake... the ability to change Itemization due to it (as long as you keep the Wind Drake Advantage).