As in all things in life....it depends.
I just have two rules:
1) State where the information came from if it is not really in your style/system.
2) Don't resort to the MSU rulebook when trying to add things. For those not familiar with the MSU rulebook, it means "make stuff up".
For example, several people mentioned adding kenpo to their TKD. That's great, but be honest where it came from. Don't take a kenpo technique and turn around and say that it is hidden within the forms or was only taught to a select few. (NOT saying this in regards to anyone in the thread, just an example, of something I have heard people say).
As for post #2, I see the trend now taking a position from a kata and then making up an application to fit a current trend and saying that it was a lost application only recently rediscovered in the kata. For example, I have seen people use the crossover step from Naihanchi and the one legged stance from Chinto katas and tell people that it is really a triangle choke from judo/BJJ and that all of the katas are meant to be applied on the ground and all of the techniques in grappling are hidden. If your school doesn't have ground grappling or groundfighting and you want to add something, fine, but be honest what you added and not lie about it, or MSU.
For me, I have enough on my plate with my style and exploration that I don't have time to fiddle around with other things. The amount of information is staggering and anything else would just be a distraction. I always think of the quote "jack of all trade, ace of nothing".