What is the advantage of the Synthesist vs. the regular Summoner? Same spells. The RS Eidolon is still combat effective, and the summoner can still cast spells on the same turn.
It removes the main weakness of the class: the summoner itself.
With the regular summoner, you target him to get rid of the eidolon. Can't do that with the synthesist.
There are a few advantages to the synthesist. The main thing is that a synthesist's defenses are much better. The combined summoner/eidolon has better saving throws and AC because of consolidated magic items and Shielded Meld. The other thing is that you can more safely focus your character's stats on casting, so you'll have more spells per day and higher Save DCs.
