"Some" people look for exploits like I said. Those people dont change how they play. Also if a rule is made then its intended workings need to be known. All I am doing is saying "how is this supposed to work". It is no more of a problem than most other rules unless you have players trying to game the system, but that is a player problem not a rules problem. If they did not want a hard rule they can just make guidelines such as "we suggest not allowing characters to take 10 if they are threatened or distracted". Making an unclear rule is never beneficial. It promotes gotcha moments from GM's.
However it is a rule and your exploit idea is ancedotal evidence. I don't think most of us play with munchkins and those are the type who would exploit things if the GM allows it.
What I was saying above was that if they issue a guideline it should clearly state how they intended for it to work not just only be a suggestion.
As an example "as a guideline you can allow the player to take 10 even if the penalty leads to failure" is not as strong as "When we wrote the rule our idea was to allow PC's to take 10 even if it meant they had to experience penalties such as taking damage. However feel free to do as you wish for your own games". The 2nd quote would be acceptable to me. The first one would not.
I don't know why you linked the gamemastery guide.