Because I don't like Rac's build. It's very good but it is not flexible. I love to play with my builds, I tweak them for almost every area and Sabway is great for that.
I dont know why that is so. Every build can be tweaked.
I dont use Rac's build in its original form for my Imbagon. I use a N/Rt orders with Splinter instead of a D/N. I have also changed the paragon heroes skills.
Although it is possible to use a triple necro build like sabway or discordway (with tweaking) with an Imbagon, paragon heroes synergize better with it. If you use a triple necro build, then you have to decide if you want to bring an orders necro. If you dont bring an orders necro, then you are not getting as much out of your Imbagon. If you do bring an orders necro, it is almost only for your Imbagon and not for your heroes, which is alittle overkill.
I dont use Rac's build in its original form for my Imbagon. I use a N/Rt orders with Splinter instead of a D/N. I have also changed the paragon heroes skills.
Although it is possible to use a triple necro build like sabway or discordway (with tweaking) with an Imbagon, paragon heroes synergize better with it. If you use a triple necro build, then you have to decide if you want to bring an orders necro. If you dont bring an orders necro, then you are not getting as much out of your Imbagon. If you do bring an orders necro, it is almost only for your Imbagon and not for your heroes, which is alittle overkill.

It's been a while but I think target-spamming helped a little. The biggest benefit of being a Nec or Mes is you get easier access to hexes/conditions so it saves skillspace and attributes on the heroes. 
