I'm, no purist, let me say that to begin. About 15 years ago I built a huge castle using Duplos as wall parts. They formed the tall and long walls, but just the brick-eating parts, all the rest was "regular" scale stuff. They are official after all. That castle is long since gone, but I'll put up a close approximation soon. (Hope I did the poll right).
Men who lie, merely hide the truth; but men who tell half-lies, have forgotten where they put it--Samuel Clemens
Since DUPLO is an official LEGO product I would use it if I had use for it. As a purist I use everything released by TLC, unaltered because TLC does not intend to release stuff to be cut and painted. So stickers that come with sets are OK. Cloth that comes with sets is OK. Strings that come with sets are OK. Even if those things ar not "LEGO" in the sense of "ABS bricks with studs".
Being a System Purist is almost impossible because every modular Castle Wall set from 1984 to 1990 has at least one grey Technic brick 1×2 with pin :-)
architect wrote:First: I believe that most "purists" would be willing to use duplo, especially for substructures. Duplo is compatible with 2x2, 2x4, etc bricks and it is made by LEGO.
Ben E. Sets Admin
That said I'll say that I have not yet found a big use for it. I played with using it for structural support in my mountain but found that the GMLTC lattice structure was much sturdier and actually might even be a bit lighter.