One of the other reasons for both a pan and a pole is that the two can't really run well together.
A pantograph requires (usually) a continuous flat wire to run under. At a switch, a new wire simply extends sideways, following the side track, and the pantograph, mounted on the locomotive, goes sideways with the locomotive, and runs along the wire that follows the track off to the side. A trolley pole can't do that. A trolley wheel/pole has a u-shaped grove that guides the shoe along the wire. When the overhead wire comes to an intersection, it needs a frog where the wire is allowed to break (just like a railroad track frog provides a place for the flange to cross over the straight rails when a car takes a siding). This frog would catch on the edge of a pantograph, and could damage the wire. Most of the time, wire is strung for either pantographs or trolley shoes, but not both.
(Think a little bit about sliding a shoebox and a hockey stick across a hardwood floor - an uneven board will catch the shoebox, but the hockey stick might slide right over the small bump.)
Also, trolley wire is supposed to be hung directly over the center of the rail, so that the trolley shoe follows the car at switches. The trolley shoe has a replacable carbon insert that contacts the wire and takes the wear and tear. Run a pantograph with the wire directly over the center of the track, and you wear a hole through the middle of your pantograph. (There's a reason they're a few feet wide. Pantograph wire is actually supposed to zigzag a little bit from side to side to wear down a wider area on the pantograph)
As mentioned earlier, trolley wire takes much lower tension (and less maintenance) then overhead wire for pantographs because the upwards force of the pole is lower then that of a pantograph.
Anyone still confused?