I am no expert and don't want to get into an all out argument, but there is a growing population that now does not advocate the removal of the proportioning valve.
As stated the Pantera valve is in the front brakes. More traditionally a proportioning valve would be in the rear braking system, to limit rear braking so the rears cannot lock before the fronts. However the Pantera front application is key to why it should be left alone in a stock braking system.
As G says the Pantera rear brakes are undersized in comparison to the front, the proportioning valve was inserted to compensate (limit) the front braking power to be more in line with the rear i.e. a matched system. When you remove that limitation front braking efficiency increases and yes the brakes feel better from a pedal persepctive, but you effectively now have two wheel braking as there is no restriction and all the stopping power is to the front brakes, you'll never ever lock up the rears because there is now close to zero rear braking efficiency. That is not a good scenario, the measure should not be just not locking up the rears, a good braking system should lock up the rears but just after the fronts.
Just my 2 cents