Mobility of any kind is extremely difficult, whether upward or lateral. Sirius and XM merged in 2008, and in the years that followed, older Millennial and Gen X employees filled the open positions. Several of those employees are not leaving the company or retiring any time soon, which means the ladder of upward mobility is in a gridlock. That becomes a problem for part-timers, who can easily get stuck in the same position for several years without career growth. There is only a certain amount of time this can last before the part-time employee has to leave for a financially sustainable opportunity. This hurts everyone in the company. Part-timers are caught between a rock and a hard place. Do you stay at a dream company and hope for the best, or do you let your empty wallet lead you out the door before you have the chance to accomplish what you really want? Upper management loses some serious talent to competitors and then must spend the time, money and resources it takes to rehire and train a new staff of part-timers in an endless cycle.
For the same reasons listed above, lack of lateral mobility brings down overall morale. At least once per week I overhear someone in the office saying, "Person A and Person B should switch jobs", and honestly, those two people just might love to have each other's positions. Consideration for an employee's strengths should always be acknowledged in job placement. When every employee is working where they can truly shine, it will show significantly in the end product.
The lack of sustainability in a part-time position is no exaggeration. Granted, it's not supposed to be a full-time gig, but the lack of proper compensation combined with the lack of promising job prospects is a double whammy.
From what I've heard trickle down the grapevine, some departments have cattiness and immaturity (think high school), but take that information with a grain of salt. Most companies have their bad apples, but my experience overall does not reflect this view. This is simply what I have heard from multiple people on several occasions.