I've noticed in ii-V-I that it is almost always ii6-V-I or ii6/5-V-I. Why is this the case? Should the descending fifth in the bass of ii-V be a very strong progression?
Outside of the V -> I cadence and the occasional Plagal motion from IV to I, leaps in the voice leading are generally in only 1 voice. Having a progression that is all root position triads and isn't planing or a simple IV -> V -> I is awkward, especially in the Classical Style that people like Beethoven, Haydn, and Mozart wrote in. Even Chopin has some pieces written in the Classical Style.
So let's take your ii -> V -> I progression. And let's assume that the V -> I is a cadence. This means that we can concentrate on the ii -> V motion. In the Classical Style, because ii is predominant and V is dominant, it isn't viewed in the same way as the V -> I cadence where leaping is encouraged. Instead, it is treated like most chord combinations, smoothest voice leading possible. I'll use the key of C major as an example here.
So let's take the root position ii. If we want this to move smoothly to the dominant chord, we would need to do this:
A -> B
F -> G
D -> D
This leaves us with a second inversion dominant triad. Now, if we are wanting to say modulate to the dominant, this could work. If we are wanting to resolve to the tonic, we could take this second inversion dominant and resolve it sort of like a French Augmented Sixth, move the sixth to an octave and keep the middle note as a common tone. But this Augmented Sixth type of resolution is not commonly used with dominant triads, especially in the times of Mozart and Haydn.
So let's see what happens when we invert the ii to ii6. Here is the smoothest motion from ii6 that is diatonic(in other words, doesn't include chords with chromatically altered notes):
D -> D
A -> B
F -> G
Aha, we have a root position dominant triad here. Now we can move to the tonic like this:
D -> G
B -> E
G -> C
leaping down a fifth or up a fourth for an Authentic Cadence(V -> I) and we have a smooth motion to the dominant AND a strong cadence.
Now, in other genres like pop and jazz, all root position is a okay. But in Classical Music, especially that written in the Classical Style, the voice leading to the dominant matters.
I think the confusion is that in English terminology ii6 means e.g. Dm6 (DFAH) while in classic harmony (German terminology) ii6 means FACD (Dm7) 1st. inversion.
ii6 (= 1st inversion of ii) is FAD ... of course! While ii56 = FACD (Dm7 1. Inversion)
As you may know this is a substitution of the subdom. and we don’t hear the fifth fall but C F6 G C (I IV V I) = a perfect authentic Cadence
There have been some good answers but since I didn't see it I'll add one more idea.
Since the lowest note of
ii6 is the root of
IV, it functions more like
Easily the most common progression especially until modern times is
ii-V-I is a very common alternative to that but it sounds different than
IV-V-I. Making it
ii6-V-I softens that difference.
ii-V is tricky because
ii is the root of the dominant of
V and in pieces that modulate you will often see
ii turning major and becoming
V/V. Also it goes into
V strongly, almost like
sol-do and that weakens the original function of resolving to
ii6 makes it sound more like
IV, because the lowest note of
ii in first inversion is
F as in
D-F-A. It basically makes it sound a little more like
IV and not resolve as strongly to
V and make the whole
IV-V-I thing still work normally by adding some minor color. The ultimate point is the function. Are we trying to focus on the tonic or the dominant?
So for the real test, play it. Play a normal root position
ii-V-I and then a
ii6-V-I and see if you can hear the difference. Then compare to
IV-V-I and see if it really does work a little bit more like that.
By the way I'm totally assuming a major key for this discussion, C major. Also I 100% acknowledge that I'm making a very subtle point and coming from a very classical point of view.
Bad data. Strong root position ii, V, I basslines are very common in many (I'd even say 'all') styles of music.
The only grain of truth in your question might be if you're talking exclusively about guitar voicings. But that's a limitation of the instrument, not a musical choice. I bet the bass player is playing those roots for you!