I think it's more irritating that there's no real reward for doing the Optional Tombs. This game has me hunting down Relics and when I get one, it shows me a friggin' Chinese coin or a Japanese fan. The game acts like it's a big deal -- there's a voiceover explaining the item and an interactive 3D model that encourages me to admire and inspect it. Yet when I complete a tomb they don't even show you the loot you just scored, or tell you what was in the treasure chest you just busted ass to get to. WTF? The game has a high level of polish, but these things make it feel like it was rushed out the door. I don't think any other videogame in the history of videogames has ever had you open a gold-encrusted chest, and then neglected to tell you the item(s) you found inside.
I thought Assassin's Creed 2 handled optional puzzle areas much better -- you got a cool set of armor if you completed them all, but the armor wasn't really necessary to complete the game. It was a nice trophy, and since it was character clothing you were constantly reminded of your achievement.
As opposed to XBL Achievements, which are clearly worth cold hard cash.