In a guild activity everyones performance is everyones concern.
It IS guild afterall.
Many guilds can't finish a crypt because too few members participate. People who don't show at all are one thing but people who swoop in to do their one hit just in case it's cleared but can't be bothered to help get the clear can be a real problem as well.
Opting down would allow those guilds some control and and plenty of players hate crypt enough that they wouldn't care about being dropped out of it or like Mitleachim (above) would jump at the chance to opt themselves out.
Why not give the guild that choice? Anyone not having a problem could always choose not to use it.