I wholeheartedly agree with everything said in this thread. To add my two cents:
As a player who recently joined a guild with a q/g system in place, the extra amount of caution you need to take is something very discouraging. Having a system in place to remove this kind of entry barrier would be very beneficial for newer players, who are still getting used to the pvp scene. And we can all agree that having more players who are actively playing pvp would be beneficial for the game as a whole.
This would probably also lead to more guilds encouraging players to do PvP, as <b> everyone</b> can easily join the fight to increase their guild rating.
It would also probably be reflected in an increase in coin sales, as people will need more bms/ms to farm efficiently, or just straight up buy coins to get a pvp ready team.
The only downside would be the need to invest time into implementing the system. This could range from simply adding some flags in the code to a whole restructure. In the long run, it would probably still be a net positive.
Implementing a System where you can choose whether or not to queue against guild members would benefit the game as a whole, and encourage more user to play PvP, as well as help keep them invested in PvP. The benefits outweigh the cost by far, and it should therefore definitely be considered.