Illusionist != healers. Let the healers worry about keeping the tank alive. Your argument for even having it is pointless. I half expect you to next defend why Oberon Ward is the greatest spell in the game since it's a 6k regenerating ward...
Dobbelganger is not a healspel and yes the casttime is to long for it to be very usefull in situations were emergency heals are needed, say if a healer drops or so.
But if you have a weak tank/healing it can be usefull on hard pulls or (with allot of luck) in other spike situations. It is just one of these ekstra things that we can trow in at a tight spot.
It is not a top priority spell but if you find it cheep then get it.
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Calaglin, Former Illusionist/Guild Leader of Dissolution on Nektulos
Calaglin, Former Illusionist/Guild Leader of Confirmed on Unrest