If I remember correctly, she gave me three reasons why you shouldn't have one installed.

1) If you pull back on the extension, you may torque the hammer and it may not cock at it's most rearward position. This will cause it to not have as much forward momentum to strike the firing pin.

2) If you pull back on the extension, you may torque the hammer and it may cock in a crooked position. This will cause more resistance on it's forward stroke and cause it to not have as much forward momentum to strike the firing pin.

3) The extension adds weight to the hammer. It must overcome that additional weight when you pull the trigger which may cause it to not have as much forward momentum to strike the firing pin.

Personally I think it is a load off cr@p. I haven't had any problems with rounds not firing except when the primer is countersunk.(Bad factory slugs) She didn't really give me a reason why it would void the warranty, just that it would.
Other than that conversation, I love my Encore and have had very good customer service from them.