It's not a true CC (it's ASDI down) but it will prevent combos at lower percents (and possibly kill you) and Marth can often ground himself at the mid percents, which opens him up to ground techs, edgecancels (depending on DI / platforms / positioning), and other nonsense. While it is true you can react and counter these, it probably would have been better if you just grabbed him.
edit:
You guys need this.
CROUCH CANCEL:
True CC involves crouching. You cannot crouch while you are in stun. Therefore, if you Drillshine Nair someone, they do not get a true CC. True CC prevents knockdown, knockback, launch, whatever for much longer than...
ASDI DOWN:
ASDI down is commonly (incorrectly) referred to as CC. It is not CC. It is fake CC. It has the same effects of CC but because you're not crouching, the effects are watered down. Don't ask why; I don't know. It can still prevent things from launching, reduce KB, and other stuff but it does it less potently than true CC. If you are in stun (by Shine or whatever) they can ASDI down.
If you do not believe there is a difference...
1) You're ********.
2) Easy way to test:
Requires: 2 controllers, gamecube / wii, SSBM, Jigglypuff unlocked
a. Open 2 players. Be Fox. Have other player be Puff.
b. Have Puff whiff Rest at 0. Hold down.
c. Semi-charge an U-smash.
It knocks her down.
Now, just calibrate the stick so she's in permanent crouch (unplug, hold stick up, plug in while holding up).
a. Kill Fox so your U-smash is fresh again.
b. Semi-charge U-smash the ducking Puff.
It doesn't knock her down. She just sits there.
Thus we can conclude there is reduced KB reduction from just holding down when you're not crouching. Why does this work this way? I don't know, but that's just how it is.