Just try me, if I don't want to to see my IP I'll just activate my tunnelbear account and write all my posts from russia. :p
Anyone else could do so, as well, so it isn't a particularly useful piece of information, except for those who don't have any reason to hide their location (most people), in which case privacy concerns outweigh idle curiosity.
IP addresses are displayed for forum administrators, but it is very rarely useful, and I barely even pay attention to user names. Sometimes spammers log in to different accounts from the same IP address, but that is something the forum can send a notification about, not that you would notice from a post itself (sometimes the second account hasn't even posted yet).