hat part-timers get the short end  of the salary and 
benefits stick is well-known.  But many may not fully appreciate the 
difference in perceived  value: as a part-timer, your input is less often 
solicited; your work is  thought to be less important, no matter what 
you're doing (your paycheque is  thought to be less important too, so you 
often have to wait longer for it);  you're automatically considered a 
beginner who needs more supervision. 

In short, if you're part-time, you don't get treated or taken seriously.
And to what do we attribute this two-class system?  An elementary but 
serious error in logic: the assumption of a causal relationship between
quantity and quality. An assumption that those working less than 35 
hours/week are not doing as good a job.

Good as in as committed?  But it's often not their choice to be
part-time instead of full-time; they'd be full-time if they could!  
And in fact, the desire to become full-time often leads to more, not 
less, commitment to one's duties.

Good as in competent?  The part-time worker is not necessarily less 
qualified or less experienced.  In fact, given the glutted job market,
the younger employees who must settle for part-time work are often more 
qualified than the older full-time workers.  (And again, they have good
reason to try harder, to be more competent.)

It doesn't make sense.  That's all there is to it.