In my old theory book, I read that while ii and ii6 harmonize scale degrees 2 and 4, they rarely harmonize scale degree 6 in the soprano. (I'm assuming they mean for common practice music, SATB). What would be the reason behind this? Are there other things to know about that might be related? For example, I've seen advice about the "II" chord (any quality) in minor keys, due to it being "diminished", where the melodic minor had not yet been introduced, so these other possibilities were not mentioned.
This seems to me to be a strange statement.
For one thing, unless the chord is tonicised as ii, the analysis of the chord as ii is not the only possible analysis. (One way to tonicise it as ii is to precede it by V of ii.) It could also be said to be functionally IV, with 2 being either a non-harmony note or a substitute note for 1.
It seems to me that the question means "When the tune has 6, why is ii rarely used to harmonise it?" -- have I interpreted it correctly? If so, then, for another thing, I'm not sure that it is rare. (I'm analysing the chord as ii anyway, for the sake of argument.)
Mozart is certainly common practice music, so I looked for examples of ii harmonising 6 in his string quartets in major keys. Here are some:
- Quartet 14 in G K387, mvt.1, b.19, 37 (3rd beat)
- Quartet 17 in Bb K458, mvt.1, b.32
- Quartet 18 in A K464, mvt.1, b.86. Also b.59, 63 unless the main pitch in the 1st violin is considered to be A rather than C#.
- Quartet 19 in C K465, mvt.1, b.40, 42, 69
- Quartet 20 in D K499, mvt.1, b.86
- Quartet 22 in Bb K589, mvt.1, b.41
- Quartet 23 in F K590, mvt.1, b.5 (after the appoggiatura resolves)
Without a reference to the book, or a longer quote, any answer is just a guess. "Old" theory books are often very prescriptive, when compared with what composers actually wrote.
That said, in common practice harmony ii and ii6 are often the start of the perfect cadence formula ii-V-I. If you don't want to make a cadence, IV and vi are perfectly good chords to harmonize scale degree 6.
Just to add AlephZero's answer the ii6 chord is the standard for the perfect cadence because it makes the big sense if you look at the bass line.
If you have the Cadential 6/4 progression you could have ii6-I6/4-V-I. That tonic chord in the second inversion is just a decoration of the dominant chord. If you look at the very important bass line you now have for C Major for instance have. F-G-G-C.
This is a standard chord progression that everyone should know for their harmony exercises.
In my old theory book, I read that while ii and ii6 harmonise scale degrees 2 and 4, they rarely harmonise scale degree 6 in the soprano
The supertonic is an excellent choice of chord after the Sub-Mediant chord.