Combined endstops for a given axis?

Why is it necessary to use up two inputs for each axis (e.g. X MIN and X MAX)? It seems like you could put two normally open switches in parallel at either end of the axis on a single input, and determine if you have tripped the MIN or the MAX by evaluating the axis stepper direction at the moment a switch is closed. What problems could arise from such a design?
a) your end stops are so close together that both are triggered simultaneously during normal printing (really unlikely)
b) your axis, through some bizarre feat, manages to travel in the opposite direction than designed during normal operation (really unlikely)
c) some other foreign object trips an end stop unexpectedly (could happen, and doesn't matter how, the print should abort)
d) ???

End stops serve two purposes AFAIK: homing which is generally done carefully and slowly before/after printing, and then for detection and prevention of unexpected damaging movements during printing. In the latter situation, maybe it doesn't even matter which end stop (MIN or MAX) is tripped, I just want the printer to stop what it's doing and abort.

Assuming the firmware (whatever it is) could use this strategy, what are the drawbacks that I'm not thinking of? It would be nice to repurpose the MAX end stop inputs for other stuff like filament detection and other safety aborts, or to even gain the ability to have a set of MAX end stops on older boards like Sanguinololu when you only have MIN inputs to begin with.

Edited 2 time(s). Last edit at 04/17/2015 04:18PM by neveroddoreven.

Most printers have at most one endstop switch per axis anyway. Some printers have less than one per axis. For example, the Ormerod and the Huxley Duo have just one switch, at one end of the Y axis, and use the proximity sensor for X homiomg, Z homing, and Z probing.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the statistics. Still, could it be effective and practical to use two same-axis end stops connected in parallel to a single input pin?

I'd think that two big reasons for not using dual end stops on an axis are a) cost and b) having run out of inputs that one wants to spend on end stops. This approach, if workable, reduces the remaining challenge down to just the cost of an extra set of end stops.