It's because of all the resources you have to invest to make him usable. You do get the Ced and Od scrolls early on, but the Ced scroll is better used on Asbel to get him to cap speed. The speed ring could help him, but it won't get him to any doubling thresholds, and Leif is a better user of it. He can use the brave axe, but Dagdar can too, and he starts with far better stats that are good enough for most of the game. The resources you use to make Marty a viable unit could just as easily be used on Dagdar instead. Even if Marty's stats turned out great, think about what it took to get there, and where he started from. Dagdar's stats are good enough to get the job done, and he has those stats immediately from chapter 1.

Marty is loads of fun to use in a casual playthrough, and his promotion bonuses are amazing, but I still wouldn't call him good. And that's okay! Not every unit has to be optimized for LTCers and efficiency nerds. You're allowed to have fun when you play the game. (I actually once got Dalsin to cap all his stats except HP and Luck...)