if the batterylife and recharge times improve and prices drop to an affordable level these bikes might be alright, the pros seem to outweigh the cons : silent, no chance of disturbing anyone, cheap running costs, crazy torque, completly tunable engine power-delivery by pressing some buttons, less moving parts that break, no stalling, no gearboxes (a pro for some, a con for others).
If one of the big japanese companies like honda would do some R'nD and present a electric dirtbike the rest of the big japs would follow too, It's the jap's that create the future of dirtbikes.