Timeless Body isn't a complete waste. It prevents magical aging effects and allows the PC to gain the benefits of aging without the penalties. This means they don't lose Str, Dex or Con while gaining bonuses to their casting stats.
It's not entirely pointless. It's conceivable that at some point in time it might, possibly, if the GM wants it to com into use.
Not likely though. Magic aging effects are few and far between, especially since they were stripped from haste. I honestly can't think of one off the top of my head. I know they exist, but I don't know any off hand.
It stops acrual of penalties, but doesn't erase them. I'd venture, and feel pretty safe doing so, that the number of campaigns where the aging rules were applied at some time other then character creation are well outside the norm. Further, I suspect that the vast majority of campaigns don't run long enough either in table or game time for them to come up unless you pick a odd age in the cusp of one of the thresholds.
To add insult to injury though, they're very high level abilities. Granted the Druid doesn't need cool abilities at that level, it's a full caster. The Monk though... he gets it two levels later and he's not a full caster.
Yeah. Ok. It's not a completely useless ability. In the finest of hair splittings. About the only class ability I can thing of that would be more useless would be "-". It's a non-ability. It's part of why they get other stuff at that level, although in the case of the monk, it's only a cool power if the DM actually allows it to be so.