You hear this all the time because....
A dealer is usually a person who gives advice either because
A: He doesn't play the game and can only rely on advice given by other players.
B: He does play the game just like a ploppy so he gives the same advice standing behind the table as he would sitting as a player.
C: He does play the game, very well in fact, knows what he's talking about, done research on the game, and can show discipline even while he's at the casino playing.
OR THE MOST LIKELY EVENT IS......
D: He may belong in either section A B or C, but he wants to be agreeable and tells you what you want to hear so he will earn his keep.
I've had to shoot a player a dirty look from time to time for jumping in mid shoe and "screwing up the cards". Of course, I know that its not true, but if I got a big tipper on the table, I'm going to do a little song and dance for 'em. If they say the table jumper screwed up the shoe than by all means I'm going to agree with them and kiss their ass a little.
Now as far as giving bad advice for the sake of this example, I usually just don't say anything otherwise if my "friend" gives bad advice. However, true, many dealers don't know what they are talking about so they will go so far as to agree with the bad advice to keep the table happy despite the downfall of one person, after all, a wrong move doesn't necessarily mean you lose that one hand.
Remember, most players, are in fact, ploppies. They really don't care much if they're ignorant about the right way to play. I can identify a player who knows what they're doing after about one or two shoes, and can usually direct that person to give the advice to the table. Anyway, every dealer is a completely different person so you can't assume anything from table to table. Identify the dealers who you like and play with them. I get my regulars who fill my table and regulars who stay far away from me.