I think most of the confusion is coming from the fact that some definitions include "working class" as being "lower middle class", so still being part of middle class.  The definition of middle class still makes sense to me, due to the socialization, spending, and lifestyle of each class.

If I try to break down my definition further, it's like so..

Type of job: Lower - part time or no job, Mid - typical 40 hour workweek, Upper - variable depending on their own ambitions.  May be 70-80 hours or nothing, but always voluntary.

Education:  Lower - high school or less, Mid - high school through professional degree, Upper - not valid to their work

Social activities: Lower - at most watch TV and do free activities, or just work, Mid - standard stuff like bars, sporting events/concerts, kids sports leagues, movies/date nights.  Upper - galas, functions, restricted access clubs, flights to other areas for regular activities.

Home:  Lower - rent in the poor parts of town, Mid - rent in the better parts or own in any part, Upper - multiple homes in exclusive communities

Childcare:  Lower - typically relies on family or one person not working, all the time, Mid - family or one person not working, but supplemented by a baby sitter, Upper - nanny

Vehicles:  Lower - one old car or subcompact, possibly shared, Mid - potentially older car, but also may have toys like a motorcycle or ATV on the side.  Usually one car per adult.  Upper - Multiple cars for different occasions or to collect them.  Potentially has a yacht, helicopter, or plane.

Air travel:  Lower - they don't air travel unless they have to, Mid - coach class, Upper - first class or private jet

General travel habits:  Lower - they don't travel, if they do it's a few days nearby, Mid - 2 weeks worth of vacation every year, may take flights or drive.  Upper - whenever they want, wherever they want, however they want

Expenses:  Lower - they are already in debt so anything on top of it is bad.  Middle - can usually swing a small expense, may have to go into debt but will eventually recover, Upper - doesn't matter unless it's 10k+

So as you guys can see, according to my definition, lower, middle and upper are quite different.  Fact is, if you are in middle class, most people you know are also middle class.  You will not see lower class people because they are too busy or can't afford to do anything.  You don't see upper class because they have their own methods of shopping and travel that won't mix with you.

If you look at an office environment, basically the lower class would be the janitor and any part time workers like interns.  The upper class is the CEO and VPs.  Middle class is everyone else, from secretary to engineer to manager.

Since middle class is incredibly massive, we can break it into lower-middle, middle, and upper-middle.  This is when you separate the managers and lawyers from the secretaries.  The difference now is in how many of the amenities in their class they can afford.  Also, they may also cross over into the upper or lower class for certain categories, such as air travel for the upper-middle (may go first class), or may have multiple cars.  Inversely, the secretary may not be able to afford air travel or taking vacations.