it seems that, in melee, the cost of invulnerability is quite high.
the perks? you do gain a unique form of movement, and, of course, the invuln.
the cons? you trade away your unlimited movement, ability to attack, and your defense until you reach the ground again.
airdodges and rolls both do this, and spotdodges plainly blew because, while they didn't leave you defenseless as much, they didn't offer much invuln either and didn't offer any movement at all. ledges nearly went this route, but rather than being defenseless attacks instead could simply finish you or, at least, put you back on the ledge eventually. the developers probably broke down each instance of invulnerability and made situational tweaks. except, uh, with the spotdodges...
like everything in melee, these things didn't obstruct the tempo of a match either since they happened quickly enough and still gave enough movement to justify making them shorter.
the brawl airdodge was poorly designed, then. someone on the melee team must've just been that brilliant.