Basically, all that I have gotten from this 901 comments long string is that Paizo doesn't care about bards. I don't know how any individual could view being able to use a bardic masterpiece that takes a few full round actions to use while simultaneously maintaining something like inspire courage could possibly be broken.
It isn't playing a wizard, a cleric, or a druid - none of the bardic performances grant anything like the power or versatility provided by the things those classes get.
Just because they seem strong or more versatile compared to some of the bards other class features and abilities doesn't mean that they actually are strong or particularly versatile in the grand scheme of things.
I understand that I am going to get a lot of disagreement about this - but if officially published material seems to make tier 2 and tier 3 classes closer to tier 1 classes, I'm going to allow my players to use it in a way that makes sense and seems fair. I'm not going to wall it away behind some arbitrary nonsense that prevents it from ever being considered useful to any character.
Hmmmm... As a PFS player that plays nothing but bard archetypes(well over 100 scenarios). I suggest that the appropriate interpretation is...
1. Masterpieces AND weird bard archetype performances that cost extra much to perform but have effects that last after actively using rounds of performance DO NOT have there effects disappear if another performance/masterpiece is performed unless it is the same one.
2. If a masterpiece OR performance has no effect after the rounds of performance are actively being used OR extended by an effect similar to lingering performance, two CANNOT be used at the same time. This rule can be violated by certain archetypes, spells, and such that SPECIFICALLY say so. That is all the proof you should need that that was the developers intent.
As to the question of a bard's power compared to other classes as a reason to allow simultaneous use beyond 1 and 2 above...bards(including most archetypes) are VERY skilled at low levels and VERY powerful in battles at high levels. There is a lot of bad advice about building fun/effective bards out there...that may be the reason you think they are underpowered. If you prioritize doing damage way more than avoiding a battle using skills, do not play a bard. If you do not appreciate that a bard's buffs(spells + performance) at level 11 and beyond will almost always make the bard the one responsible for the most damage, do not play a bard. A bards best spells and best performances are buffs. A bards best class features are all about skills. Lean too far away from that at your own peril.