I think Minato is summoning the original to give Naruto the Yin chakra.
In this thread i am going to explain how i think sage chakra and kyuubi chakra could work together, but why it right now would not make naruto stronger and thus it would not be beneficial to do so.
lets assume these are the chakra levels:
Current Kyuubi=50 (misses yin)
Naruto=6
Controlled Kyuubi mode benefit at the time he met pain = +6
Sage mode would make you 1.3x as strong:
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Okay, so my idea is that the SM+KM could only workout for so much is because he needed to form his own chakra into yin chakra to compensate for kurama's yin being missing.
It would work out as something like this:
6 controlled Kyuubi chakra + 6 Naruto Yin chakra = 12 chakra + 6 sage mode chakra = 18 chakra level.
Once naruto has the yin chakra of the kyuubi he could use it like this:
100 full Kyuubi chakra level (including yin) + 50 sage chakra level = 150 charka level.
So to conclude this theory: the reason naruto is not using kyuubi+sage mode atm is because it would require him to act as yin chakra (wich is obviously ALOT less than just using Kurama's yang).
Can't wait untill Minato hands Naruto the yin chakra and find out what kind of crazy power moves he could make! ^.^
TL;DR: naruto will get 3x the amount of chakra than he currently has according to manga facts