A lot of this depends on the head you are starting off with and what gains you are expecting to get.

If you use this alone as a method of tuning then you will see very little gains. But if you are using this along with other modifications then it will help. Being able to flow more air through the head is always a bonus, as long as its done properly.

I would never take a head off just for porting and polishing, but if I was changing cams and valves, then you may as well have it done at this point because the cost isn't a big issue then.

May as well optimise everything if you can.

But if yoy took the head off purely for porting and polishing, the costs would outwieght the benefits IMO.