This one comes up a lot...
When I talk about LT those are the numbers that I mean. You are 100% right and the sports scientists are always talking about the technical LT -- however -- what we really want is an estimate that works in the field. These two methods are effective.
A well trained, rested, highly motivated athlete will be able to hold LT for 60 mins in a race situation (elite half marathon, say) -- however -- that's not really applicable for the general population. Even then, I believe that his training zones would be best determined by the method above.
Of course, a lactate step test is useful for additional information but I've found that the answer is rarely different than above. The information that's most useful from lactate analysis is confirmation of AeT as well as knowledge gained from a review of the athlete's lactate curve.