Desecration is terrible, even without the glyph of Scourge Strike. Every PVE Unholy DPSer has Epidemic. Epidemic increased the duration of your diseases. Epidemic does not increase the duration of your Desecration. You will be Plague Striking only once every 20sec, which means Desecration will be up only 12/20ths (60%) of the time. This means that Desecration is pretty much a flat 3% damage buff, which, for 5 talent points, is very, very hard to justify.
Moreover, it will be buffing the "setup" part of your rotation, and will never be up during the 3xSS and RP dump... which means that even the 3% damage buff estimate is generous.