Source isn't on the cutting edge, to put it mildly. It's continually updated to meet the needs of the games they release, but Valve's priorities are very different from those of Epic or Crytek.
Case in point: their toolchain remains good ol' Hammer, vis, rad, and bsp. They're stretching lightmaps to the limit with a toolset from Quake that's constantly being pushed. God knows how long it'll be before the returns from refactoring start diminishing... it'd make a good case study, actually.