1 - if you like to RD from elsewhere, its probably helpful to have a static ip even if using some dynamic dns service i think static ip better. Likewise if run a service of any kind like email server etc.
2 - tbb as already mentioned.
3 - if you have ACL's on things you access (which are ip based) or you audit thigns you access meaning its easier when its a static ip, it helps on that as well.
4 - dynamic ips can be flooded with torrent traffic or other rogue traffic depending on what the previous owner of the ip used it for, likewise could be banned on sites etc.
BT Infinity 2 Since Dec 2012 - Estimate 65.9/20 - Attainable peak 110/36 - Current Sync 71/20