They should be limited by rule of no free PvP. You may only target another player with offensive spells if he agreed to it. Also i think Defensive Wind can be cast on yourself only because you need to channel the magical energies after the spell is cast, and so only the caster can control the wind around him (my explanation at least)
I wholeheartly agree that you should always be able to target yourself, even with offensive spells.
(off-topic) Defensive Wind should either need no target, or a /target_me command must be implemented. It is a pain to cast currently, especially when you use (like me) only first person mode.