I would think they'd be useful up close, like at a cabaret style club where you could be sitting only feet from the stage. Some clubs have the speakers hanging over the stage and facing down/out into the crowd, so a wide cardiod could sound great up close, even stealthed I would think.
In general, I doubt you'd stealth the MK21 nearly as much as the MK4 or MK41, but I guess it depends on the venues you go to. If the Bottom Line were still in business, I think the MK21 in a hat would be an awesome setup to use there.
- Jason