Why not A6? No reason, as far as I'm concerned. I probably use it more than anything when I'm playing without pedals. These days, I often play a fair amount of Western Swing material, where A6 really sounds and feels right to me and I use the 5th on top a lot. Since I play pedal steel, I'm really not trying to emulate pedal steel when playing without pedals, and I tend to find the high G or G# a bit washed out on C6 or E13. If I'm just carrying E9 pedal steel, I often re-tune the bottom two strings and press A&B pedals to get a pretty complete no-pedals A6 with the root on the bottom string.
And the 6 string version, well I just prefer to have the root on the bottom string.
Yes, I agree. A 6th tuning that doesn't go down at least to the low root feels pretty limited to me. And A6 without the high E (which, if included, puts the 3rd on the bottom string of a 6-stringer) is missing too much of the high end for me. My response has basically been to ditch most of my 6-strings.
On 8-string, I often re-tune between A6 = (lo-hi) F# A C# E F# A C# E and C6 = G A C E G A C E, which works perfectly with the same string gauges. Advantage A6 when going to A6/F#7 = F# A# C# E F# A C# E, which puts the root on the bottom string and the normal 6-string A6 on the top 6 strings, as opposed to C6 on only the top 5 strings for C6/A7.
I like the low tunings. My 8-string resonator has, nominally, G6 = E G B D E G B D. Tuning the low G to G# to give G6/E7 = E G# B D E G B D gives a nice twist. As a guitar/slide guitar player, this doesn't feel too low to me either.
Of course, this is all subject to personal preference and what is being played.
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