[Azriel]

Actually, I'd say a reservoir is important for a few reasons.

A pump submerged in a reservoir is MUCH quieter.
It allows you to visually inspect your coolant, as well as your flow and level.
More water will allow your system to take short periods of extreem load better. This being applicable only when the system is absorbing more heat that it can dissipate.
10 second bleeding.
Higher flow rate. the flow of any mag drive pump will suffer greatly with a hose attached to it's inlet.
Easy filling and draining.
They look ****ing sweet.