Bloody hell there are some depressing old fashioned views on this thread.
I work part time. I'm worth investing in because I'm very good at what I do, and if they don't continue to invest in me, I'll leave and it would be their loss! Like any decent employee would be.
Thankfully my company agree, and on return from maternity leave both times, I've had training / progression opportunities.
Part time workers are more efficient and often more loyal (as it's often hard to change to another part time job). Not giving them the same training and progression opportunities as full time staff is discriminatory and short sighted - helping employees stay engaged and skilled and achieve a better work life balance leads to better results for the company, less stressed / sick employees etc.
Holding people back because they are part time leads to resentment and good people leaving.
I would say that as my company have treated me well, I am flexible in return and will work at the weekend to catch up etc if there is something urgent on, respond to urgent messages on my day off if I can etc (though it's not expected).
I think flexible working will increase in the future