Because I don't think what works against Vegas is going to work as well against Colorado, who's the likely 2nd round match-up. If you let them have the puck the Avs can hang 2-3 goals on you very quickly, and at that point you need to have more than one line that's able to create offense (especially if Ek's line is in full time shut-down mode). If Game 3 ends up being relatively low risk and that line isn't generating offense anyway, it's a good time to take the chance.
Plus despite all my hedging I do have a high degree of confidence in Boldy as a player. Whether he can get it going in 1 game or 5, I'm not sure... but I don't think it'll take him long.