Thanks for the thoughts everyone. Seems like most teams aren't savvy enough to coordinate, or have enough people online that they can just ask for heroes if needed. But portal difficulty 6 sounds like RNG is often a factor. (Though guilds who haven't hit the level 6 portals yet might benefit more from sharing since they may be stretching their resources just to level up their core teams, much less some niche heroes for portals).
Its better to have our own heroes, cause for outland portal you will need several tries to beat it, especially Phoenix portal (with WD), and Lord of caves.
For Raid sometimes people don't want to put their top damage heroes, personally I think heroes that usefull for raid, like for example chap 13 and 14: savage one, ninja, or SM need to be in mercenary. this will increase members damage thus shortening the raid, means short time on gear queue.
I usually put CW and SM all day. Then Switch to Pilot and Phoenix on Dragon day as Phoenix and flying portal are the 2 hardest. My guildmates can complete the rest pretty easily.
You always have to put in your best heroes in mercs, even if not hired, they generate most coinage.