i only have this on in the bookmarks. it does not explicitly say such but its something along the line. I have a dedicated G network. Since all my network use the same password, people may accidentally connect to the N network.
The article does not say that one should use N-Only, it does suggest that one try not to use B/G clients on an N network...
Greenfield (N-Only mode) is impacted when a G-client is seen - even if that G-client is part of an overlapping BSS - If an N-only AP sees it (and it has to scan for non-HT mode BSS's) it will invoke protection modes to minimize impact on the legacy mode BSS - and this protection, for lack of a better word, is expensive.
A mixed-mode HT (802.11n) uses different protections, namely the legacy headers, which is less expensive in terms of time.
In any event, the current crop of 11n/11ac chipsets are pretty clever about things - they take the user setting as a preferred mode, but generally will make up their own mind - most will start in mixed mode anyways, and then, and only then, if they do not see any legacy BSS's, they'll shift modes... and automatically shift back to mixed if a legacy BSS is seen, rather than go into full blown Greenfield protection.
With a AC class AP/Router, they're pretty smart, and you'll have less impact of a G client jumping on to it, then you would with a dedicated B/G only AP just for the G-clients. There's a definite benefit to moving all clients to N obviously - single stream USB dongle can be found for less than $20 to retrofit older laptops and desktops.
Unfortunately, there are two popular gaming consoles that are 11g - PS3 and the Wii, along with every Nintendo DS handheld (with the exception of the DS/DSLite which as 11b/WEP only, so time to upgrade the kids there if you still have those two models around).
Key thing - and this goes for both 11g and 11n - really don't want 11b anywhere even close, as they will force the entire BSS into ERP protection, and this is a big impact... huge actually, and trust me, just don't bring 11b clients into the mix if at all possible...
We've many cases here on the forums where the collective advice has been to move to mixed-mode when helping out 2.4GHz 11n, and the results have been good.