The Problem:
Sometimes you don't want the PC's to find everything in a room. If you're running a game set in a castle, involving courtly intrigue and a number of assassinations that begin occuring as the game progresses, then the fact that there are a number of secret passages riddling the castle walls could very well be a big deal. One that, while they should be found eventually, shouldn't necessarily be found right away. I've had at least one situation similar to this occur.
Search can really be a big deal in a Purloined Letter or, well, Clue-esque game. Sometimes, Search shouldn't be a guaranteed or zero percent chance of success thing.
Now, while you might just say that in instances like that, involve some kind of roll, but that doesn't work so well. If you do that, things look suspicious.
For some games, that might not matter. Then again, it very well might come up at a later date. At which point, you're essentially screwed. Sure, you can add in random hazards or what have you but, at least in the examples I provided, that doesn't necessarily make any sense.
I don't have situations like that always crop up, but because I want to have situations like that, and have had situations like that, I've altered the taking 20 roll; not just for Search, mind you, but all skills. Even when taking 20, a roll has to be made. On a 1, taking 20 only equates to taking 15. Rolls need to be made until no more 1's come up - so potentially, that 20 could turn into a 15, or a 10 or even into the negatives on a really bad night.
All around, I don't like the "automatic success" or "automatic failure" of the taking 20 rule.
On top of that, I also occasionally like for the PC's to enlist the help of outside parties; not not necessarily because the PC's can't do something, just because that trying so should be exceedingly difficult for their skill level (but doesn't necessarily entail any risks for trying).