Deepwater Gauntlets  

 

Most click-effect items, especially older pre Luclin ones have casting times significantly longer than the spell they are based on.

It's a trade off for not having to commit the spell to memory and use mana to cast it. They also generally require you to be higher level to use them, than you can cast the spell at.

Enchanters get Cancel Magic at level 8. It requires 3.5 seconds to cast, has a 5 second recast time and costs 30 mana. (everything the same except level for other classes)

If you find a mob with a nasty damage shield on it that's going to make a difference, you could sit, memorize cancel magic, then once it is ready, you can cast the spell, wait and see if the DS is gone, then re-memorize your other spell later.

If you have a clickable Cancel Magic, you can click the item, and as soon as you're done casting, you resume your other activities.

If you're quick, you might be able to drop a spell and memorize cancel magic in about 1-2 seconds. It will take at least a second then before you can cast it. You're up to 5.5-6.5 seconds to memorize and cast. Then you've used 30 mana, and have to change out a spell later.

I've had several clickable items I use instead of a similar spell I can cast myself. It all depends on the circumstances.

If you have the spell memorized, it's more time efficient to cast from memory. Meanwhile it's more mana efficient to cast it from an item. So in that case, it comes down to whether your time spent using the item is more valuable than the mana used to save that time. (Which in large part is decided by what you can do with the difference in time -meditate, nuke, melee, etc.)

It's also very convenient to have some of those clickable effects around when you don't have reason to expect a particular effect will be needed, but it turns out that, surprise, it'd really help.