With the greatest of respect, and with my corporate strategy hat on, the MPL, as I understand it, is a method for increasing corporate control over a company's pilot workforce and thereby reducing terms and conditions, thus making it very desirable. It's about lowering the market value of pilots, thats all.
As I understand it, the MPL is specific to an aircraft type and configuration, and specific to a company. What that means is that if you leave the company, you effectively have no licence anymore, and thus sharply reduced bargaining power.
Of course someone else can hire you, but then they have to "convert" you to their standard of MPL.
In other words, you are locked into your employer and change is very difficult because your skills are not easily transferable....or at least thats how I understand it.