I take the position of agreeing with the 2008 code on this.
On the face of it, the primary hazard exists with line-to-line loads. You need a means to disconnect all ungrounded conductors into a load to avoid cross energizing. It might seem that line-to-neutral loads are safe in this regard.
However, maintenance of (parts of) a MWBC could involve disconnecting the grounded conductor. In the case of single phase 3-wire, this can result in a hot neutral. In the case of three phase 4-wire, this could create the additional hazard of voltage instability between equipment on the other still-connected phases. Note that this rule applies to the means of disconnect, not the OCPD. It's the disconnect that applies when maintenance is in progress.
I'm sure no one here would ever do such work without disconnecting all the phases, especially if the work might need to break the neutral during progress. But are all the phases even going to be next to each other when single pole breakers are used (on a panel you didn't wire up)? Requiring a common disconnect ensures a grouped disconnect. It also ensures a condition safe for maintenance being applied to the ungrounded conductor (or any conductor).
