To be honest I have no idea how any "serious" Star Wars 1/6 collector would not have the Emperor in their collection.
I think it all depends on the collector's own preference.
I bought the head and the cloak, and built the ROTS throne(From the Invisible Hand), and that will be more than enough for me. I have never seriously been tempted to buy Vader, and i can't really explain why...
I have Anakin, Obi-Wan, and Maul, but for me(Although i was born in 76, and have been a fan of the OT my whole life, probably like most of you guys), the main original Trilogy HT character's have seldom moved me to buy them. I have ROTJ Luke, TESB Leia, but nothing else. Although i could be tempted by a TESB Han, i'd be on the fence.
You can be a serious collector, but not be a completist with every character, lol.