Since I think a lot of this is about me, I think I'll throw my 2 cents in the ring.
A good friend from RRBC messaged me asking for me to play in a premade the following weekend in a role that I do not prefer, play often, or am known for playing. I did not look at the event as being a long term commitment to RRBC.
However, I have to honor the fact that they essentially purchased a character for me using funds from THEIR guild. If I'm not a member of their guild, then my toon certainly is. And since I can't give that character to one of their accounts, that character is inextricably bound to be played by me.
I had reached a prior agreement with Living to be a long term member of his guild community, and I am obviously going to honor that commitment. It's not a question of obligation there; it's a desire to play with the people who I perceive to be the best, and who are willing to take the time and currency to fund a character for me.
I think the take away here should be that we as a community now number a good handful of guilds, but have less than the total number of skilled, cooperative players required to consistently fill a premade team for each guild. If players have friendships in other guilds, then they should be allowed to have a toon in the guild and participate in some of their events. Ultimately what it comes down to is which guild the player will prioritize above all others; and that's his main guild.
The impetus should be on the guild leader to recognize that a player he substitutes might have responsibilities to another guild, and will not be able to participate as a regular, core member of that premade team.
Of course, this should be analyzed on a player by player basis, and good communication should happen between all 3 parties (Main Guild Leader, Substitute Guild Leader, and Player). If I'm asked not to participate as a substitute in any premades by either Guild Leader, then that should be the end of it.
Tl;dr - Give help when it's needed, but be careful how many promises you make - you might not be able to keep them all.