Do you actually have one?
In fact there are a broad range of accessories including a whole variety of abrasives. One has to shop around to find the best prices, which are not at all expensive.
If you are using a Rotothingy to do anything other than very crude hacking of holes, then you deserve the results that you have experienced with cutting pipe.
I find that I use my Multimaster very regularly. In fact it hardly goes back in the box.