Buffet made his fortune buying underpriced assets... and he is NOT against buybacks. He IS against buybacks of stocks that are selling for more than one and a half times book value.... and usually only 1.2 times bv... ie he is against purchasing overpriced assets.

Why "one and a half time book value?" Probably because of earnings visibility. If you don't see competitive threats arising, then your investment horizon is perhaps three to five years... which is enough time to raise the current tangible book value by ~ 50% (or more or less). This is likely only true in a sector where there are high barriers to entry. 

I gather his theory is that in the inevitable situation where new competitive threats will impact the companies' ability to earn a profit. If the competition is serious enough, it will impact the share price... and the floor to that share price is the tangible book value (all other things being equal) minus foreseeable bottom line losses.