Two reasons often-cited for having spoilerons:

 - better roll control (especially during stall)
 - doesn't take up space for flaps

For the MU-2, the primary reason was to allow for full-span flaps at the rear, lowering clean wing area while still having acceptable stall speeds in the takeoff/landing configurations (and therefore shorter landing distances).

However, spoilerons have the disadvantage of killing lift, which can be deadly in situations such as a one-engine-out situation in a twin. To compensate for this, usually minimal constant spoiler use is recommended, and mostly rudder is used if an engine fails. If available, aileron trim is used to reduce drag if long-term use is expected.

Spoilerons tend to have less of a difference in roll rates at low and high speeds, compared to ailerons -- this could either be considered an advantage or a disadvantage, depending on the situation and whether a pilot is used to one type of roll control or another. This can negatively affect the roll control feel of the aircraft though, as it might feel slightly sluggish at higher airspeeds. They also tend not to provide much (if any) feedback to the pilot, which is a disadvantage.

Note that most large airliners use both spoilerons and ailerons for roll control, to be able to still have an aileron while supplementing large control inputs with the spoilers.