Ok this is some napkin maths might be wrong but meh, btw the times were taking from a previous casting/spell speed analysis.
But the 30% buff selene provides is effective, even more so when you dont need Eos.
-30% on a base BLM spell speed 341 will provide a total casting reduction of .54secs which brings Fire I to 1.96 cast.

Lets take a 30 second period of uninterrupted casting -
Without buff 12 casts will = 1800 potency
With buff 15 casts = 2250
that's a 450 potency boost lets say 2 Blms 900 potency boost + the affect on others. Please note that is a boost over 30 secs, when considering the whole fight it would be a significant boost.

There is no reason not to use Selene when Eos isnt specifically required.