for a player phase murderer, swordmaster robin is just fine, the only thing Id think about is you lose the ability to really have 1-2 range weapons on robin anymore (barring the levin sword and stuff like that) but if you need a boss killer and your team lacks the single target damage that astra can provide, then go for it! Swordmasters are cool anyways, hell I beat this game back when I was a young lad who thought knights were the best things ever and reclassed everyone and thier child to a knight if they could.... the game was slow but I still enjoyed it.
But yeah robin swordmaster can fill a niche your team needs but he will suffer as a generalist in combat compared to a strategist who can use magic for 1-2 range, but if your team needs it its worth it still, and if you think its cool then go for it.