I personally think multi-rail PSUs for computers is a marketing gimmick. The idea for multi-rails is to provide isolation between the various systems supported by the PSU. That may be fine in big, commercial or government communications facilities, but not in a single computer. If you lose the rail to your graphics card, for example, it is not like the rest of the computer will take over and continue to support your computing tasks.
Multi-rail PSUs are MUCH more complicated, using many more components. For this reason, as Thrax noted, single rails are more reliable.
Also, on multi-rail PSUs, there typically is a reserved amount of power that always remains available on each rail - even if not used! While on a single rail system, all the power is available all the time. So a 650W single rail PSU can provide more power on the single rail than a multi-rail 650W PSU can provide on any one rail. This means in some cases, a more powerful (read: more expensive) multi-rail PSU may be needed when a less expensive (and potentially more reliable) single rail would do.
This does not suggest multi-rails are unreliable - at least not those from reputable makers. But the potential for component failure with multi-rails is higher.