The call to add low-level flying capability came out of fears that Russia would field increasingly more advanced radars that would neutralize the B-2's low observable attributes. As such, being able to sneak in below radar via flying nap of the earth flight profiles like its B-1 brethren became a requirement for what was a high-flying strategic bomber that relied primarily on its stealthy design and careful mission planning for survival.
Not only did this useless addition make the B-2 less stealthy from certain aspects and against certain radar bandwidths (most notably from behind against longer wavelengths), but it also added weight and dropped the aircraft's altitude ceiling substantially, from roughly 60,000 feet to under 50,000 feet. The change also impacted the design's range and most of all it added substantially to the cost and complexity of the already very high-end and expensive aircraft.