Actually, the rules that introduce flaws do specifically state it. From Page 91 of Unearthed Arcana:
"Flaws are like the flip side of feats. Whereas a feat enables a character to be better than normal at performing a task (Or even to do something that normal characters can't), a flaw restricts a character's capabilities or imposes a penalty of some sort."
"A player may select up to two flaws when creating a character. After 1st level, a character cannot take on additional flaws unless the game master specifically allows it (For examples of times when doing this might be appropriate, see Character Traits, above). Each flaw a player selects entitles his character to a bonus feat. In other words, when you create a character, if you select two flaws, you can also take two bonus feats beyond those your character would be normally entitled to."
The balance justification for this is that a flaw is not simply the inverse of a feat, but rather worse. Taking a flaw and a feat that affect the same thing results in a net loss. For instance, the Shaky flaw mentioned before hits the character with a -2 penalty to all ranged attacks. The Weapon focus feat on the other hand grants a bonus of +1 to the attacks of a single weapon.
This means the only good reason to select flaws is if the feat obtained is more valuable to the character than whatever is penalized, though Dungeon Masters are encouraged to ensure that any flaws taken are not simply bypassed.
By all means, it is your decision, but if the only reason for denying it is its not written in the rules, well, that is not quite accurate.
Edited: daishain on 17th Apr, 2017 - 1:43pm
Wow, I didn't remember that. Thanks for the reminder for me personally. I've been in games where Dungeon Masters didn't allow them. Now, it's been years so it may be because that was in UA. My memory gets foggy… that may be a flaw! ;).
KN, would it be okay with you to do as Ninja mentioned? Take shaky and change name to near sighted? Then I can take a bonus feat.
Okay, I'm starting to get a little lonely in that cow pasture. :P If anyone needs assistance getting ready to start, by all means, let me know. I'm quite willing.
Out of Character: What you do not like hanging out with a few cows? I have heard they are fun when trying to walk all over you once they knock you off your feet.
I think Den is ready to romp through some cow patties.
I activated gems. Can you look and okay everything. I haven't picked for my third gem in case you have something in mind for that one… especially since it kind of mirrors a Ranger ability.