The goal of reducing lower rates is to reduce channel utilization, that is IF you have high channel utilization. A side effect of doing this properly is that roaming can improve, as well.
You first should start considering removing lower rates if your RF utilization on the Aruba Controller Dashboard is at 20% at Midnight (low to no users on access points) in a dense deployment. During the day, you do not want your utilization at 50% if you have few or little users on, either. It is quite possible that you could go to 70% utilization during the day, but that could be because of congestion or interference, so you would probably want to measure it at night so that you can measure the next night after you remove lower rates to see if that solves your issue. If you have Airwave, you should also be able to tell historically if you have RF utilization at 20% at night by looking at the b/g radio of an access point and going back to midnight and looking at the utilization graph. In addition in Airwave, if more airtime is spent "Receiving" under channel utilization is another indicator that management traffic, instead of data is consuming airtime and you should consider trimming rates. You should enable Drop Broadcast and Multicast as a prerequisite to removing rates to ensure that is not the issue.
Once again, you would only consider trimming rates if you have (1) High Utilization and (2) a dense deployment. Once you trim rates, you effectively reduce the radius at which clients can connect. Management frames at the speeds of 1, 5, 6, 9, 11 transmit further than the rates of 12 and above, but they consume more airtime. I would not start with removing rates if you have a sparse deployment
With that being said, 90% of the time you would consider trimming rates on the b/g (2.4ghz band) side, because it definitely has more clients, interference and fewer channels to service them with. If you want to consider trimming rates, everything below 12 (removing "b" clients effectively) is not an unreasonable place to start if you have a dense deployment. To remove everything under 12, these are the 3 parameters in the SSID profile> Advanced that you would change:
If you have 3 SSIDs being broadcast in an Ap-Group, you would have to change this on all 3. A packet capture in the Air should show that all management frames are now being sent at 12 to confirm...
"So let's assume I've Walked the site floor and we get >11Mbps everywhere. On that basis, the proposition sounds solid. However, the gap we potentially create, is isolation of let's call them "weak" client devices. I.e. those with terrible drivers/nics or antennas. I.e. we're elevating the playing field for the greater good, but potentially discriminating against the weak. Be mindful this is a BYOD scenario, so we're talking about all manner of "uncontrolled" devices (from an RF point of view)." - The Racking.Monkey
Do not make decisions based on if you get 11Mbps everywhere, because the negotiated rate of a client is determined by a number of factors that you do not have direct control over. Use the utilization number at midnight, because that is something that can be measured after the change...
I hope this helps.