The attitude seems to be "I never saw it before, it's different, therefor it must be somehow wrong." Don't blame the world for your lack of experience!

Whether or not to 'share' neutrals is a design issue, up to the discretion of the installing electrician. Regardless of voltage ... as long as all the 'hots' are fed by different legs, and the legs are of approximately the same voltage to ground, the use of shared neutrals is perfectly proper.

Can things go wrong, especially in the hands of unqualified personnel? Absolutely. That's part of the reason I prefer to avoid using them in homes. Yet, my preference is far different from being a code requirement.
Likewise, it's nice that all the hots terminate in breakers mounted next to each other - but, again, that hasn't been code.
Handle ties? Generally, IMO, a poor idea. It's really nice that ONLY the faulted circuit trip; this helps identify which circuit has a problem, and you're not shutting off the lights when a appliance served by the other leg faults. This is but one area where the 2008 NEC is causing some unhappiness.

Yes, the 2008 NEC greatly changes the way the code looks at multi-wire branch circuits. I consider that discussion to be beyond the scope of this thread.

So, why would an electrician chose to use this method? Primarily, to make room for more circuits in the conduit, and to reduce the clutter in the panel.
Limiting ourselves to Romex as the wiring method under discussion (since that is the primary residential method), running two separate 12/2's brings 6 wires into the panel - two hots, 2 neutrals, two grounds. Use 12/3, and there are now only 4 wires .... 2 hots, 1 neutral, 1 ground. This also means an easier pull (round wire vs. flat), fewer staples, less attic clutter, etc. There can be some cost savings as less wire may be used as well.