First point kinda yep. You will probably waste more time moving the line that somebody else caring about its position.
In my second point _(int)$mybb->input['uid']_ may not be set if for example users visit:
http://mybbhacks.zingaburga.com/member.p...http://mybbhacks.zingaburga.com/member.php?acti
Third point is about the correct code and hook to use to make your plugin more friendly (using the correct hook on case, somehow, an user is created outside the registration page and a referrer is set, for example) and use less code.
Fourth point will probably not make a difference if you use Google SEO but core SEFs are.. different:
http://community.mybb.com/member.php?act...http://community.mybb.com/member.php?action=profile
Pretty sure somebody will complain eventually for this.
Lastly, send_pm() is just about using less code and it also helps you if you want to send the message in the user's language instead of the board default.
I wouldn't just call them "preference" but "experience" I have obtained from writing a bunch of free and private plugins, simple and extensive ones equally.