You'll open your wallet again if it sinks to $3 or $4. That's the type of investor companies love, ones who open up their wallets and donate more money to them regardless of results.
PPS is a poor indicator of the ultimate directions of a company. If I followed your criteria I would have sold Ariad when it plunged to 2.50, (I didn't) then it sold for $24 two years later. The significant variable with respect to buying/selling is your confidence in the ultimate value of the product.