I can give TEN reasons why I sometimes shutdown or ignore the /a channel:
1) Someone got on my nerves & I need a time out.
2) Busy with personal pursuits.
3) Just want quiet.
4) Have nothing to offer to the conversation.
5) Don't want to announce my presence.
6) The message was a general announcement or greeting.
7) Already in a private conversation.

Folks are monopolizing the channel with tells better sent via PM.
9) Spammed with combat text.
10) Any combination of the above to some degree or other.