Yes a CORE group is is that, people that progress through raiding. Personally I feel that if you join a guild and the guild changes it's mission statement or reason for being then that is a good reason to leave. I hate guilds personally, I am tired of bickering, lack of consistent behavior and constant disagreements or changing of minds (such as Tier 15 core groups having to assist Tier 14 people on a whim). That just benefits lazy people and in the end core group people are being punished for their persistence.
If it weren't for the Guild perks I wouldn't be in a guild at all, period. There are PLENTY of good PUGs out there, I know.. because I have had some really good random groups, the problem is we can't ALL be on at the same time, that's the REAL issue. Life is what's happening outside of WoW which impacts players and the quality of guilds.
What you are left with is 15 year old's who have nothing better to do that have ZERO clue about how to run a guild they are there because they can do it 24/7.. sadly that's the truth! You should NOT think a guild is run with in effort, it's there to cater to the people that can play ALL the time. People leave guilds, people come and go and it has always been this way and always will.
People are the PROBLEM, not guilds, not WoW, and certainly not content. Why do you think the majority of games are SINGLE player? Yeah because online games are REALLY hard to get cooperation, developers see $$$ signs, but with the exception of WoW no one makes any money at it... it's a advertising draw... People are fickle never satisfied never content, its whats exciting NOW, tomorrow it's going to suck again. Not surprised people leave guilds and come and go like the wind.. that's the nature of people as well.