^Hmmm...personally I think that Disc 4 was necessary. Firstly, I dont think the leap from saving the world to saving the Universe is actually that huge. Either way they're both massive, but once it gets to that scale you hardly notice the difference between the two (to an extent).

You say that it should have ended at Terra, but I would find that ending too rushed. Also, the assimilation would still have continued, and there would need another portion dealing with stopping that - and it would most likely be anticlimatic.

Personally, I dont think Memoria and Necron did that much to the overall storyline. It just introduced new ideas which i thought fitted in with the general idea. Memoria also provided some excellent boss battles.

I do agree though, I dont think Necron really was necessary at all - in fact, it was pretty strange. But either way, I dont care too much.