When is it worth to use entchanted reprise?

[MaraD_]

The Swiftcast Verslowspell happens in both scenarios, so let's ignore that. Then the cost of the E.Reprise, besides the time taken to use it, is just the two spells totalling 20 mana. It doesn't require an additional generation of the X 'lost' mana that 'would have been generated' during the time I spent executing E.Reprise.

Its still lost mana.
its like using manafication at 50/50.
you gain 100 mana, as opposed to using it at 40/40, which you only gained 80.
The extra 20 lost, is still mana you will eventually use, even if its not in the very next WS combo.
(and the amount of seconds you waste waiting to use manafication also lowers your DPS per second)

if it didnt take up the GCD, THEN it would be a completely different story.
It takes up;
Mana generation
Mana (consumes it)
GCD

Its the very reverse way in which Swift cast is a DPS gain.
Swift Cast is only a DPS gain, because slow spells do more dmg per GCD.

If you spend that GCD not generating mana for the actual spender, but instead waste it towards a weaker spender, you lost 3 things, not simply 1.
(technically you save on MP, until lucid is up, but MP shouldnt be an issue. Moulinent (w/e the cone is called) is better for taking up less time in the GCD, and more potency efficient.)
Only if u must stay in range, and cant keep casting, otherwise you have better alternatives.