I can understand when someone is busy and have plans. However, I know a few people who pick and choose when to say this to certain people just avoid hanging out with them.
Any other word besides YES really means No. I am dead serious about this. It could be "maybe," "we'll see," "I'll let you know." My problem is that maybe you do have to check with your personal secretary, but it's up to you to get back to that person to update your status on the situation.
I find it funny that people could find the time to go online to check their e-mails, update their status on Facebook and even comment others, when they claim they are so "busy" that they didn't have time to update you.
Everyone needs a break from life but do do make time for what we want to do in life.