This isn't a clear cut debate and many witches and wiccans have very different ideas on the morality of binding. Strictly speaking a binding spell is cast to prevent somebody doing harm.
The idea is that you prevent them from completing actions that harm others by restricting their behaviours. Its kind of an aggressive protection spell, instead of shielding yourself from being harmed, you stop the harm from being started.
Tricky morally. You could argue that by binding someone you are preventing harm and therefore your spell is justifiable. But equally you could say that by binding you are taking a person's free will to act away and that is potentially as harmful as allowing them to continue doing whatever it was that they were doing in the first place. (Not being a Wiccan I didn't feel I could advise on the morality in relation to my friend's religion but it certainly got me thinking about the morality in relation to mine.)
Binding spells are one of the most useful spells in a witch's grimoire. I've got lots of little plasticine figures tied up in cord and sealed in little bags. I bind for a variety of reasons. Firstly it targets the specifics. Maintaining shields strong enough to withhold any negativity requires a lot of energy. To eliminate the threat at source with a simple binding is a lot less effort. Secondly pure defensive protection on myself still allows the perpetrator of the harm to do it to somebody else. Thirdly, if somebody annoys me I'm going to retaliate with any tools at my disposal!
I don't personally have the issues with binding that I have with hexing which I use much more sparingly. Though I do tend to use hexes rather defensively as opposed to going on the attack. I created a serenity spell recently where I sealed my serenity into a witch bottle and then hexed the bottle so any deliberate attempts to cause me stress would backfire onto the person causing it. Rather a natural selection sort of a hex, only invoked when malice was employed toward me.... So far only a degu has irritated me - and she very quickly stopped squeaking, ha!
Of course, binding doesn't just have to be about preventing harm to others. In a sense with my serenity spell I was binding an element of myself. I was sealing it away and binding it closer to myself. I've seen binding used in love spells to bind people together (now there is a sticky morality wicket...) And I've seen it used as an act of love to stop people turning on themselves. Its quite common when binding somebody to include wording to the effect that they can't harm themselves or others.
I'm a big believer with spell work that the spell lasts while the effort is maintained. So a spell for a one off incident can be cast and forgot but for an enduring spell like a binding spell, if you want it to last indefinitely it needs to be energised regularly. I think that is why I don't really have a moral problem with it. If I could flick a switch and stop a person forever that would bring the responsibility of too much power. But as soon as the focus is taken off a binding spell, the person gradually returns to their behaviours. However, sometimes a break from them is all that you need and the binding provides the opportunity for exactly that. Equally if a person is likely to cause harm in a given situation, it may be prudent to bind them until that situation has passed.
I don't advertise to people when I have bound them but I have watched their behaviour afterwards with interest as the spell seems to take effect in very different ways. Sometimes people seem to voluntarily change their behaviours and sometimes they try to continue being nasty but don't have the strength to succeed. And in one exceptionally successful binding to a rather horrible individual who attempted to manipulate everyone around them, the spell worked by everyone else simply not taking notice of them any more.
I think it is important that you bind alongside other protective magics such as shielding and strengthening yourself. Sometimes once you have shored up your own strength you may even find that you don't need to bind, that whatever threat you felt the person represented, it isn't as viable when you are functioning at full strength. And any spell is always more likely to yield good results if you are in good spiritual health anyway.
To conclude, although binding has a few grey areas and certainly raises a few moral eyebrows in some circles, in my own opinion the benefits are too good to worry excessively about the moral angle. Besides, if the person you intend to bind was doing nothing wrong in the first place, you wouldn't need to bind them, would you?
Image http://www.sxc.hu/photo/983241 (Cobrasoft)