There used to be a person in my guild who would say hello to everyone who logged in. Quite a few people in the guild, myself included, asked him repeatedly to stop because it was unnecessary traffic on the channel.
Another thing to look at is the fact that you could be AFK while your AddOn does the talking for you. Someone could respond to you and wonder why you don't answer them.
I'm with Torhal. I don't like it when people greet me (or anybody) every time I log in. It gets especially annoying if I have connectivity issues or am logging all my characters to check my mail or auctions.