PTSC's cash could grow in value at 3% to 5% annually without much risk (CDs for example).

Stock could have less value than stock; $1 worth of stock may not be viewed as worth as much as $1 cash to a prospective M&A target. (Could also go the other way).

So the expected stock appreciation has to be greater than the safe cash appreciation to make stock buy back a logical choice.

IMO, if there is not a buy back that says there isn't enough stock appreciation expected, compared to safe rates of return on cash.