*is a Marth player too <.<;;* You may have already said this, but this only works on Lucas and Ness, right(And only Lucas up to 20%)?
Here's some advice from a Marth mainer, by the way. I'll only speak from how I play, though, as I don't know how everyone else plays against a Lucas. I, personally, wouldn't risk trying to get a grab in place for just 20% damage. I could easily accomplish more than 20% with two tipped f-airs, which really aren't that hard. I'm an aggressive player like many Marths but I don't just run around with the c-stick and Dancing Blade. I aerial almost always for spacing reasons and tip all the time with f-airs and b-airs. That means I don't want you close to me, I only want you my sword's length away, and that's it. I won't grab you unless you're close enough for me to, and then I throw you either into an -attempted- combo or to the nearest edge and put more pressure on you there. But unless you try a dash attack as the first action in the fight, I won't grab you (shield grab > obvious dash attacks). Grabs are much more situational, and I'd only take them when I can get them. That being said, I also grab a lot, as shieldgrabs are much more useful in Brawl than in Melee.
But: Now, a Ness player who can't escape from this well I would completely abuse this as much as I could. I wouldn't be stupid and ONLY go for grabs, but I'd probably rack up as much damage anywhere in the battle with this "chain grab" and wait until they're at a decent percent. Then, due to his stun after the grab, a tipped or even non-tipped f-smash would do for an easy KO.
Watch out though, if you're afraid someone will take the f-smash after the grabs, DON'T SHIELD IT. Spotdodge it. If you shield it, someone can STILL grab you, if you spotdodge, you're safe. And if you hold the shield you can c-stick a rolldodge away from him, and then pressure him with long range attacks (if you like, I don't know your playstyle).