Many people will talk about how it will impact the person's chances of getting a job and all that.
How could furthering one's education negatively impact chances of a job? I think those people that are saying this may have taken a Master's degree and are annoyed that their airline did not recognize their achievement.
I have a Master and PhD within aviation and management; that is, no engineering stuff. No airline has ever regarded my further education negatively.
If you are already on the Citation and can do your flying without disturbance while studying at school then go for it. One day you may be able to take on additional responsibilities with your new education. If you are doing it simply to better yourself then don't worry what others may think.
I currently work at a university so I am a little biased when it comes to recommending higher education. Having another degree never hurt anyone, and you never know when it may come in handy in the future; that job in the front of the plane may not last forever...as I found out.