A lot of what I have done has come from
Designing Great Beers, which examined nineteen recipes (eleven of which were all grain) that had made it to at least the second round of the NHC.
The vast majority of those all grain recipes used three or four (usually four) base grains - 2 row, pilsner, Munich, Vienna. The percentages of these base grains varied wildly in balance and such.
Base grains represented 77-92 percent of the grain bill, with 82% being the average.
The vast majority used crystal malt, typically in the 10L - 60L range. Many recipes used more than one crystal.
The vast majority used a small amount of carapils or wheat. A few used both.
The average mash temp was 152.5 degrees F. None used decoction mashes.
Caramunich, melanoiden, and aromatic mals were used fairly regularly. One brewer used a small amount of chocolate malt.
I'm all for tradition and simplicity. I don't doubt for a moment that you can make an amazing ofest with 2-3 grains. I could see the fun in trying to brew an amazing beer on your skill alone with a simple ingredient list.
That being said, these beers - which largely had 5+ grains - were judged to be the best representations of the style at NHC. Heck, Biermuncher's massively popular recipe (which, I will grant, is an ale version) has seven grains in the bill.
I don't think that it's fair to generalize that a grain bill that is more complex than you might choose to brew is automatically an inferior beer.