They're different forms of defense: Evasion + high Reflex saves provides a way of avoiding dmg from anything with a Reflex save (i.e., traps & spells); while PRR/MRR provides dmg reduction on (almost) anything which hits you. You can read the
wiki page for more, but a simple comparison at lvl 28 (presuming Div Power or Tensers to max BAB):
lt armor: 15 + (28 / 2) + 2 = 31 PRR (~23% dmg reduction)
hvy armor: 45 + 28 + 6 = 79 PRR (~44% dmg reduction)
So right off the bat, hvy armor provides almost double the dmg reduction of lt armor; though that gap narrows with additional PRR from gear etc. I think the max combined PRR bonus you can get from gear is +55, though that includes a Planar Focus of Prowess which req's using a CitW weapon. So best-case scenario on the build I posted upthread, let's say +55 PRR from gear, +10 PRR from Imp Parry, +10 PRR from Imp Shield, and +25 PRR from defensive stance.
lt armor: 31 + 55 + 10 + 10 + 25 = 131 PRR (~56.5% dmg reduction)
hvy armor: 79 + 55 + 10 + 10 + 25 = 179 PRR (~64% dmg reduction)
Unyielding Sentinel in Unbreakable stance would be another +30 PRR, which would be ~61.5% vs ~67.5%. So it is still possible to achieve high PRR in lt armor, but as you can see it takes a considerable investment, which involves sacrifices elsewhere.
MRR is another matter, though, because for some reason a lot of PRR bonuses don't boost MRR as well; it's also designed to favor hvy & tower shields, which is part of why we're seeing a resurgence in classic S&B builds. [The other part is the DPS gains from pally & Vanguard, ofc.]
tl;dr summary: Evasion is still a good way of avoiding a lot of spell dmg (provided your Reflex saves are high enough, ofc), but it doesn't do a thing for physical dmg nor non-Reflex-based spells. PRR/MRR mitigates dmg from almost everything; and a hvy-armor wearer gets better protection with less effort than a lt-armor wearer. As an
added incentive, Shadow Striker is only available on med or hvy Shadowscale armor; and Shadow Guardian is only available on hvy armor.