"I suspect very, very few people today use Duotone, Tritone or Quadtone...
As for size, the TIFF and PSD limitations are AFAIK"
You see duotones, etc. in higher end printing when designers want a certain look, and while they may not be used as much, it's something that is still done more often than you might think. It's definitely an art to getting it right but when you do, it really does look great. And just because it's a feature that might not be in wide use or one that the current crop of designer know little about doesn't mean it shouldn't be supported.
As far as size limitations, they are not the same. psd has a 2 gb limit while tiff has double that - a 4 gb limit, further making tiff the preferred file type as so many files exceed 2 gb these days - well, at least here they do.