I the last year I have flown in various cabins with five different airlines. Only one of them identified the location of emergency exits by reference to row numbers. On a widebody with fixed cabins, it may be best to that there are exits at the front and rear of each cabin section. But on a single-aisle aircraft without hard dividers it would be very useful to identify the exits by row number as well as by (usually ineffectually) pointing to them. Most people know which row they are in and can do simple arithmetic, so working out whether forward or backward is the best direction, at briefing time, is quite easy - much easier than counting rows when the cabin is full. of course in an actual emergenc it may be better to follow the crowd and head for the popular, rather thna logical, direction.