MK1 ARB drop links were the best mass produced solution at the time for Mazda to get the cars selling at the price they were aiming for at the time.
The MK2 onward utilised rod end ARB drop links. This was a direct development to the already fantastic MK1 suspension set up. By then sales were exceeding all expectations so a little additional cost was not going to put anyone off.
Rubber bushes act like an elastic band in the ARB set up of the MK1 so cause delay in the action of the ARB. Incorporating rod ends as Mazda did from MK2 onward got round that problem.
Having adjustable ARB drop links means that any preload in the ARB can be adjusted out for a neutral Anti Roll Bar when the car is sitting on level ground with any load on board ie Driver and passenger with luggage for the weekend, or driver only with the car interior stripped out for track use.
MK2 onward you would only need one ARB drop link adjuster per axle so as to be able to dial out any preload.
With the MK1 you need only one adjuster per axle as above but you need to get ride of the elastic bands on the other side of the ARB`s. so a full car kit is required.