In the case of the MPD's the Swing feature is a component of the Note Repeat.
So, you add swing to the midi notes that are spit out when Note Repeat is active. So swing only applies to when you are using note repeat.
Have no idea what the exact practicality of that feature is. I suppose you can add swing to hi-hats if you like to use the Note Repeat to sequence your hi-hats.
But if you have a piece of software (i.e. Ableton Live can do this, and I think Cubase can do this, among others) that can extract grooves. You can use the MPD swing function to get the MPC/MPD grooves and apply said grooves to your sequences when you quantize or add groove to a MIDI track.
The MPD's are great pieces of MIDI Controller hardware. Unfortunately, they are poorly implemented by Akai (as is the trend in the majority of their products these days).
A vast majority of users find that the 32's pads aren't sensitive enough, so you have to hit them hard to register a decent hit. The workaround that people have come up with is to open up the unit and apply layers of tape to the underside of the rubber pads in order to increase sensitivity (there are also Pad Corx, and other Pad Sensitivity Fix kits that you can find over the internet if you don't want to use tape).
Another issue involves the Note Repeat. There are sync issues. I've owned the 32 and found that I was unable to consistently get the note repeat to sync properly with software. When set to external mode, I've never found that it works. When set to internal mode, it is shotty at best. Sometimes it works sometimes it doesn't, sometimes the sync works for a little bit and then goes, often times the timing of the Note Repeats are completely off; rendering it useless. Although, I purchased the 32 around the time it came out (and returned it within a week). Its possible that Akai has fixed the issue in subsequent firmware upgrades. I also owned the 26, and found that the issue was still unresolved in the 26 (note repeat does not sync properly).
Another issue, involves saving Global Settings. Last I heard, the MPD's do not save Global Settings after you power down. Even though this is supposed to work and saving global settings is given as a feature on the Unit, people have found that their settings are not saved after they have powered down and powered back on again.
So just keep in mind that those are potential issues that you may come across and have to deal with (that is until Akai potentially comes up with a fix and solid firmware updates). Knowing Akai, it could take a while to get the MPDs rock solid, and its likely that Akai just might never fix the issues.
Additionally, it's evident that there does not exist a forum dedicated to MPD users. A real shame, considering there are a lot of MPD users and there are still many issues with the unit, but no central source for fixes, workarounds, announcements, and shared intel. If anything it would be a nice support group. Hopefully someone starts a forum soon.
I'm not sure if there have been fixes, since I've done my research into the MPD line. If there are any, I would like to know, as the MPD's seem like a great idea but poorly executed by Akai.