BAC ACOGs have lit reticles bright enough for CQB, with both eyes open, therefore negating the need for a seperate optic to address the deficiencies of a fine crosshair reticle scope such as the TA01 series.
However, the beauty of an Optima 2000 or Docter sight on top of the TA01 is that each scope can be sighted in for different ranges, the red dot for 25yds, the ACOG for 100yds, for instance. With a single scope you have to compensate when shooting at short range targets by shooting a few inches below the intended point of impact.
Drawback? Taller profile, and no cheekweld when using the red dot, more like chin weld.