It somehow sounds like your sales guy should stick to sales. It sounds
like he is worried about the fact that the exhaust is going up as opposed
to over. I have heard that one before, but given the the exhaust is warm,
it naturally rises and you could need less, not more effort to exhaust out
the roof straight up than the same total distance in any other direction,
especially if other directions required bends.

True, the warm air will rise, but it won't have enough force to take the
lint with it. Only about 70% of the dryer air goes through the lint filter.
The rest is allowed to bypass the filter...I assume to prevent problems if
the filter is full. This air carries lint with it and will quickly clog the
vent line if the line is too long or too high in the air. The suggestion of
a booster fan in the line would probably help. I sold dryers in AZ where it
is common to run straight up through the roof and I postponed the
replacement of many dryers by telling the customer to simple clean out his
vent line. It would usually be plugged right where it made it's first turn
up into the wall. The worse was of course when the builder put another 90
degree bend in the ceiling to go horizontally out a side wall. Really hard
to clean that out.
Tom.