the presense of peat smoked malt in a scottish recipe is something that actually causes debates. some prominent folks say it is not true to style. I really don't care what those guys think, though.
Peated malt has a very STRONG flavor. It only takes a couple of oz to get some flavor in the beer. Too much of it and it would be overpowering and the beer would not be enjoyable.
The first version of the recipe had only 2 oz or so, but I bumped it up on the subsequent batches until I got it where I wanted it. I actually think I put 4oz in the last batch I made, but never recorded it. I am using 8oz for the 10 gallon batch Sunday.
It makes the flavor of the beer a little more unique, IMO.