If you're only doing 2 miles at a time then cheaper trainers may be ok but if you're out doing long, regular miles you may get injured using cheaper trainers. But people in these shops know their stuff and their advice is valuable and they should be supported; they can pick up pronation/supination issues or other musculo skeletal problems that you weren't previously aware of and by so doing advise you to get the appropriate treatment or suggest other orthotics or trainers depending on the severity.
