Kudos to the writers for finally explaining that confusing bit from Get it Done, concerning the Shadow Men, and their offer of Demon Power.
Do you remember how after that episode everyone on the Buffy Boards was going crazy trying to figure out if Buffy should have accepted that Demon Power.
A lot of people thought she should have. Buffy didn't, because according to her Buffy logic, "No! because it's wrong".
Now it turns out the Shadowmen became the Watchers. The Watchers (as we know) cannot be trusted. Buffy was probably right not to accept the power.
Also do you notice Guardians appear to be female. Watchers (or at least the power structure of the Watchers) appears to be male. This is a show about girl-power.
Why did the Guardians hide the scythe from the Watchers? They didn't trust them (and who could blame them).
The scythe was an anti-doomsday devise reserved for the slayer alone (thus when she touches it feels like hers to her). The Guardians entrusted one of their own to "guard" the secret of the scythe until what she judged to be the true "end of days". Why did they do this? If you could hide the secret of the atomic bomb, would you do it?