If not for stamp dealer letters, there would be some missing unique postal history items ...
Here is a modern dealer cover from 1993.
It contained Dan Mayo's auction catalog mailing, a 7 ounce rate during the 2/3/1991-12/31/1994 period:
0.29 for the first ounce
0.23 x6 for the additional ounces
Exactly totaling 1.67
Is it philatelic? Shades of gray.
No doubt some will emphatically say "yes" because it was sent by a collector/dealer, regardless of any other factors.
And no doubt some will say "no" equally as loud since it was paid with a minimum number of current definitive stamps conducting the normal business operations of a company with no particular philatelic intent.
What if this envelope were franked identically and sent by a local plumbing company with their supply catalog?
Regardless of the camp you fall in, this cover is set aside to "age and mature" for the next few decades like a Prexie or Liberty series postal history item with an unusual franking. Show me another like it!