ARU
Why send an ARU?
It depends on the force area I guess, but in most they double as traffic units since they are usually pursuit trained and the cars are up to it. Since they often have a lower workload it makes sense rather than having them sat around the station waiting for a comparitively rare (in most areas compared to traffic) firearms incident.
As for the tasering, I'm curious as to how you'd have attempted to stop a JCB. It's not like a tactical stop is going to work, or stingers, or boxing it in. And it would be too dangerous to jump into the cab - all it would take is a size 10 in your face to throw you onto the tracks. It's probably got a decent sized fuel tank so can keep going for a reasonably long time, and if the drivers already swerved at the copper, who's to say he won't start swerving at pedestrians or drive into someones house? The officer was probably hoping that once he'd been tasered he'd have given up. It's a logical assumption.
And of course tasers aren't replacements for firearms - they have an effective range of around 5 meters. They fill a similar role to the H&K Baton gun but on a more general issue, IE they provide a less lethal option for use generally on suspects armed with a weapon other than firearms, who would previously have been shot. However the intention is clearly to slot them below this even - Baton guns require you to be firearms qualified and then (in the MDP at least) do an additional course to qualify on the Baton Gun itself.
I'm assuming the officer believed that after being tasered the driver would give up. It's not an unreasonable assumption in my mind - if someone shot me with a taser and I knew they could do it again and again, I'd stop what I was doing and comply. Wouldn't you?