Why dealers disparage certain products?

It is an individual style, choice, and decision they may simply be expressing they're sincere, genuine, honest opinion and perspective even though you would never do such a thing.

You do not raise yourself up by putting someone else down.

It is the sign of a fool. Keep moving.

To OP:

I completely agree, Roy.  You and I shared the same employer for several decades.  Bad-mouthing the competition is a losing strategy, but all too common.  Passing on anecdotes about the reliability or lack thereof of opposing brands is another favorite ploy.  No one can expect any dealer to have every line of product nor to have experienced what they don't deal in in a meaningful way.  But customers about to spend big bucks want to be convinced they are making the best choice.  Positive persuasive techniques don't always do the job, so the temptation to go negative is always there.  Witness the recent contretemps between Upscale Audio and Raven regarding their respective products.

@clearthink - he's in the business of selling gear to customers; I'm not. It's his livelihood, not mine. As a potential customer, if I don't like his 'individual style', I will respond to that style by applying my own individual style, which would be walking out of his store and never coming back. 

What about @hilde45 fanning the flames on the McIntosh fans?