The article suggests that the suppressor was developed primarily to quiet the blast while employing it as a breaching weapon, hence the built in standoff device. However, it also suggests the benefit of reduced muzzle flash (in night, low light engagements by SWAT/tactical officers) as well as cooling and dispersing the muzzle gases, reducing the propensity for the muzzle flash to ignite flammable materials and vapors in drug lab raids.
Why would one carry such a weapon in place of a MP5SD or other silenced/suppressed weapon? I would wager for the same reason many choose the shotgun for close range work, its terminal effect. If one is employing subsonic ammunition for maximum silencing effectiveness, other silenced weapons give up the high velocity impact that gives their projectiles much of their wounding effect. Bullet construction can compensate to a point, as can special weapons firing extremely heavy bullet/low muzzle velocity cartridges, ala the SSK .300/.338 Whisper series of weapons.
A subsonic shotgun round (nominally 9- 00 buckshot pellets at 1050fps) still has more KE than any silenced cartridge that comes to mind. To compensate for energy loss due to the reduced velocity, they could up the weight of buckshot and slugs. The article also states that the even the supersonic rounds did not alert people standing 25-30 yds away, giving the same terminal effect with greatly reduced noise.
I'd imagine that if one could get comfortable with the extra length and change in balance due to adding the suppressor, the extra weight and internal recoil reduction that is seen with suppressors would allow for rapid follow up shots.