Yes, they are really optional. Interacting with those certain guilds is as optional as the addons they require.
In fact, their requeriment is also optional: they could have decided not to require them.
The only way the addons would be really not optional would be if the whole or at least the great majority of the roleplaying community decided that people not using that addon should be shunned off and ignored. And if the community does that, then there's a much bigger problem than the addon's existance: that the community is composed of jerks. The addon would just make it explicit.
In fact, their requeriment is also optional: they could have decided not to require them.
The only way the addons would be really not optional would be if the whole or at least the great majority of the roleplaying community decided that people not using that addon should be shunned off and ignored. And if the community does that, then there's a much bigger problem than the addon's existance: that the community is composed of jerks. The addon would just make it explicit.