Joe -
You asked if 'this' was a way of saving money....I'm sure that may play some part with this practice, but if you check out 310-15(b)(4)(a) [1999] this will give a good reason for using the shared neutral. Example: if you tried to pull 6 circuits in a conduit using your one-neutral-per-circuit method you would have to derate the conductors 50% (Table 310-15b2a)... #12 THHN would only be rated for a 15A circuit ... using the shared neutral process you only derate 20%....in fact, you could get nine 20A circuits (using #12THHN) in one conduit (shared neutral method)vs. only 4 circuits using your method....god I hope I didn't blow my arithmetic!
