You are already at 5K?  Wondering why have you pulled that many Sacreds?

The answer to your question depends on how many more you have to burn and how badly you need one Mythical Book.  I wouldn't do it, but I have 25 Books and only Fjorad's Alt Form and Anaxia unbooked.  All my other Mythicals are fully booked.  

I have pulled 5 so far and have 41 left.  I have pulled a lot of greenies to fet to 5500.  I could pull another 500 greenies.   

Embrys needs 5 more books.  I have 4 plus this one.  Mikage and Nais are fully booked.  

It depends on the value of Embrys to my teams.  

Personally I wouldnt.  Odds are there will be one in the Titan event upcoming that I think you'd be better off using those sacreds toward.