Borrowed time is meh even if you have a lot of haste, unless its bonus haste like heroism. I posted the detail calculations in this forum somewhere showing that there is no real benefit to stacking haste instead of crit. However you always have some haste from the haste buff. Even 2% extra haste on 3 PoH casts is enough to make it a net gain especially since PWS overheals less than PoH in general.
Even if its by a tiny amount PWS:3xPoH is better HPS and better HPM than straight PoH spam.