Using Faster Casting and Spell Echo in the same time?

Hi,
I've been looking for Arc Witch build and most of buids has Arc+Spell Echo+(...).
I've read some threads and I know that Spell Echo is just better than Faster Casting.
But I'm wondering why in this builds no one uses both of this gems.

There are two reasons.

1. Faster Casting is mostly not the best choice DPS-wise. Your tree usually has enough increased cast speed for you, as does your wand.

2. Mana issues/effectiveness. With the MORE cast speed of Spell Echo AND faster casting increased cast speed, you do cast very fast, and that drains your mana at an incredible rate. Further, most mobs you encounter don't require a lot of spells to die.

It depends on what you want to accomplish also. In Ambush I had an Incinerate witch that used Cybil's Paw and I used both Spell Echo and Faster Casting. Cybil's Paw gave me life on hit for spells so the more (faster) I hit, the more (faster) I'd recover any life lost. If your build has on hit effects, Faster Casting can be a good choice, especially with Spell Echo's multiplying of it. But as previously noted, you will spend mana faster. My build could run max level Arctic Armour so standing still while casting gave me plenty of regen to handle it.

Keep in mind that Spell Echo provides a MORE bonus to cast speed. Because it is a multiplier rather than an addition you need some cast speed to begin with to multiply out. Now you could just grab cast speed nodes on the tree but you'll spend 2726472652287027 points trying to get half as much cast speed from the tree as you can from a single 20/20 Faster Casting gem. Thus the two gems work together. Spell Echo increases the gains you get from Faster Casting.

That said, if you already have a goodly amount of cast speed (e.g. from your weapon, jewellery and a few points which are close to your chosen path on the tree) you can often get a better DPS return on your gem slot investment by choosing a different support.

The reason that Faster Casting is used in Arc and Incinerate builds (which naturally grab as much cast speed as possible) over other supports is the dmg effectiveness of those spells. Only getting 50% value from another gem like Added Chaos or Added Lightning really sucks when you can get 100% value from Faster Casting since it is not adding flat dmg but instead upping your rate of fire.

Edit: I think you'll find that a lot of Arc builds -DO- use both gems for the reason I outlined above.