You have wheels and wheels but it depends too of the rider level. Marketing does a lot too, if one company tells you that you will get like 50 MPH faster with their set people will buy them. IMO the shop was just making a point but it depends on the wheelset too.

If you are just the casual rider that goes to work using a bike and during the weekends rides like 50 miles at 15 mph average, then pretty much any wheelset will do as long it is in good condition. If the guy is a racer, then they will get the faster, lighter and strongest wheel in the market because sure the cheappo wheels the casual rider is using wont work more than a week w/o having to repair them.

For example the big M have wheelsets for regular people that are pretty good at that level (this is when marketing kicks in), for serious racing those called "reliable wheels", wouldn't stand a chance of even been in the list of possible brands/models considered for a team. So it depends, IMO the guy at the shop was kind'a making a point. At the same time, if the rider sucks, that rider even riding a bicycle made of AIR with zero drag the guy will suck.