I might have used that term because one hears pipers doing it all the time, but rarely hears whistleplayers or fluteplayers doing it. In fact, I can’t offhand remember hearing a whistleplayer or fluteplayer doing one though I certainly don’t have Total Recall. The term “pipers’ roll” is one of convenience but might not be a term used by pipers (except me). Perhaps “double-pat roll” would be more suitable (we have “double-cut rolls” after all.)
Obviously on the whistle one can do an ordinary roll anytime, two in a row on the same note, with a bit of tonguing separating the two. But when a piper tongues it doesn’t have much effect on what the chanter is doing
so doing a “pipers’ roll” for the second one is a digital solution to the issue:
whistle: G(A)G(F#)G ’ G(A)G(F#)G
pipes: G(A)G(F#)G (A)G(F#)G(F#)G
(the gracenotes are shown as neighbor-tones as a shorthand but in fact might be more distant notes)
But pipers do them all the time, not just in such situations. The famous example is Paddy Keenan’s beginning of the Kesh Jig on the Bothy Band album
(A)G(F#)G(F#)G
They can be, and are, done on any note an ordinary roll is done on
(A)E(D)E(D)E
(A)F#(E)F#(E)
etc.
Here is Paddy Keenan doing them on G and A (not all the time! but most of the time)
http://www.youtube.com/watch?v=-bbw1nSGtoE
Sometimes pipers extend these with yet another pat, for example starting Kesh Jig
(A)G(F#)G(F#)G (F#)GAB
Is this the only sort of roll pipers use? Of course not! Most of the time they use ordinary rolls, some of the time these “pipers’ rolls”, and quite possibly other sorts of rolls from time to time, depending on the player and the musical situation. Sometimes, on a series of repeated notes, they might continue a sequence of alternating cuts and pats, with the cuts and pats ending up in unusual spots (not where they would be expected in ordinary rolls).
An example could be yet another way to start the Kesh Jig
(A)G(F#)G(A)G (F#)GAB
which is fairly common.
I will add that there was an old Clare fluteplayer here, Ray Tubridy (cousin of Michael), who would do an interesting variation on the note G in many of his tunes: he would park on G for several notes, all these Gs being separated by a series of pats. He didn’t do double-pat rolls as a usual part of his playing.