Russell was moving away from any sort of inter denominational Christian fellowship by the end of his career/life. Rutherford just continued the process to its logical conclusion: That being the total destruction of any form of ecumenical feelings between the Bible students and all other Christian denominations. This was done to demarcate the separateness and exclusivity of the Bible students.
That same attitude is cultivated in Jehovah's Witnesses today. If Christians have a practice or a new trend then Jehovah's Witnesses are automatically suspicious even if the Governing Body has made no specific pronouncement on the matter. Why? Because the idea of separateness is what counts and correctness or being reasonable is secondary. Case and point: Clambake just posted a topic where his wife and the congregation held a party on January 2 but she could not go to the Nov 15 end of year company shindig. Its all about separateness. The idea that you're in a new spiritual Israel and like the Israel of the OT you have to keep away from everybody and everything because Yahweh will have a conniption fit if you don't.
The practices the Witnesses object to are actually irrelevant. Its just done to contradict and disagree with every other denomination.
BTW I love your avatar and the word ecumenical, therefore a quote by Jack Sparrow: [to Weatherby Swann]
"I think we've all arrived at a very special place... spiritually, ecumenically... grammatically."