But Dratini gains experience slooooowly. Your Dratini will be half the level your starter could be with the same experience, at least early on when the starters level like nothing.
And the late evolutions? You'll get your Dragonair when most Pokemon are already into their third stage. It's a tremendous stat disadvantage.
Lance wasn't kidding when he said "they're hard to capture and raise"--dragons (legendaries excepted) make for an incredible challenge early on. It doesn't seem like it'd be that way, because most of the time you can't get them until late in the game where there's already a fabulous team to support them in battle.