The presentation that you refer to is mine. The reason to set the beacon rate one rate higher is so that the AP is a good roaming candidate. In other words, if the beacon rate is the same as the lowest supported data rate then it is feasible that the device could see it as a roaming candidate, connect to it and then have to roam immediately off of it because it was on the edge of coverage from a power standpoint. This would especially manifest itself where the AP power is slightly too high because the client might put it higher in it's roaming table. In the real world if someone has taken time to trim their data rates then I would hope the power is tuned appropriately as well so this is really a non issue. Either way, it's going to work just fine, you'll just have unnessary roaming if the power is slightly too high and the beacon rate is at the lowest supported data rate. I hope this makes sense.
BTW, I've attached an updated presentation that includes speaker notes, Airmatch slides and some updates to the client match slides.
Best,
Andrew Hejnar
ajh@hpe.com