I need a clarification between these two answers: 1 2
The first answer argues that Shield Master removes only the penalties from attack rolls made with a shield. So for example a character that has the Shield Master feat, has a Bonus Attack of +6/+1 and is armed with a longsword (primary) and a light shield (secondary), his full-round attack would look like this:
+4longsword/+6lightshield/-1longsword
or
+2longsword/+6heavyshield/-3longsword
with a heavy shield.
The second answer, which is corroborated by an official FAQ answer, seems to argue instead that Shield Master removes all attack roll penalties made when attacking with a shield. So, again, a character that has the Shield Master feat, has a Bonus Attack of +6/+1 and is armed with a longsword (primary) and a light shield (secondary), his full-round attack would look like this:
+6longsword/+6lightshield/+1longsword
or
+6longsword/+6heavyshield/+1longsword
with a heavyshield.
Which would effectively make the feat Two-Weapon Fighting (although essential to the acquisition of Shield Master) and light shields' powerplay (except for the fact that, unlike heavy shields, you can hold small objects with a hand equipped with a light shield) obsolete.
A third answer (which I came up with, based on the fact that a lot of people think that corresponding primary and secondary attacks happen at the same time) may be that Shield Master removes all attack roll penalties, but only when attacking with a shield. So, same character with Shield Master, Bonus Attack of +6/+1, armed with a longsword (primary) and a light shield (secondary), his full-round attack would look like this (the first two attacks happen at the same time and a shield is involved so Shield Master is applied, the third one is believed to happen separately so it doesn't benefit from the feat):
+6longsword/+6lightshield/-1longsword
or
+6longsword/+6heavyshield/-3longsword
with a heavy shield.
Which one of these outcomes is the correct one?