I greet EVERYBODY that comes into my room at this point-- still easy as I don't get massive room counts. To me, its just good manners and it isn't something I can help. Some say hello, some instantly leave, some sit for 3 hours nights on end and never speak a word. It is what it is.
I don't go to MFC, I use only CB so I am unsure how it works on other cam sites, in terms of how many sites/tabs/windows you can have open at a time.
On CB though, you can have as many open at once as your computer can handle. So I think what many guys do, is open multiple tabs of models to see where the action is, or what fetish/kink they are into at that moment and which takes their "fancy". So they may just leave you open. It happens to most models. I think some have more malicious intent, too (personal recordings for example).
Some models will kick those that stay there and don't talk, especially if they notice the same person/people doing it day in, day out.
The other day I went into a room where a model had the setting "must have tipped in the past 24hrs in order to message" and whilst I was reading her notifiers for a few rules and other things - which literally took 2mins - she PMed me and I tried messaging her (at this point I wasn't aware of the fact I could not message without tipping) and as I was writing out a brief "hello" message (She had asked how I am and I am someone who hates typing "hru" and similar stuff, I will write full words such as "I am fine thank you. How are you this evening?".. anyway I sent that message it said the error and then 5 seconds later I was banned from her room haha. Not even time to process the message or anything else.
I think that's partly CB's fault though, if the model PMs someone and that rule is being applied, then either 1) The model needs to receive a warning saying "this member has yet to tip you, thus can not reply to your PM", or 2) Simply allow - in PM only - the person to respond, since the model has initiated the PM.