I've seen it too, but not many mechs can pull it off, or it can't be done in too many terrain types. Sure, you can get a really tough fast mech in close, and if it's a really high end mech (Clan especially, like a Stormcrow B or Gargoyle C) it might have enough power to hack into the side torsos before either the Devastator or it's lance mates bring the fool down, which needs to be damn fast because the first turn the Devastator wins int it just walks back three hexes and fires the big guns at short range with no mods.
In tight terrain, you can get a King Crab or Sagitire or Kodiak or Turkina D in close, with the power to just blow the damn thing to hell free and clear. It's easier for the jumpers, of course, and an ultra high end Clan mech like a Dire Wolf C can just shoot it's way in close, but for a King Crab or Hunchback you need just the right terrain, and if one takes a Devastator into terrain where that can take place, then they deserve what they get.
Honestly, I think the minimum ranges on PPCs and GRs are more dangerous to opponents than to the mechs that wield them. I've killed more mechs with Awesomes who thought they could get in and exploit my minimums, than with foes who thought they could stand back and slug it out (though granted no one imagines they can just slug it out with an Awesome).