I can't really speak for any member of staff but myself, but if we allowed posts in other user's Request Threads, it would honestly turn the Request Thread forums into a mess. We allow "once-per-twenty-four-hours" bumping to give everyone a chance to have their thread on the front page for an amount of time; if we allowed users to post in other user's Request Threads, someone could just recruit their friends to bump their thread at all hours of the day, and then those who are new to the site or don't have anyone available to do so wouldn't have a fair chance at keeping their thread visible.
As for advertising roleplay in status posts/chat, if we allowed that, things would be like F-List, constantly spammed with RP ads instead of being what they are really for: community engagement. We would rather people come into chat to talk about their day, about their interests, have conversations with people; and we would rather people use status posts to do the same: post funny things they find to make other people laugh, talk about something cool that happened to them that day, etc. If we allowed roleplay ads there, it would become entirely overrun.