There was a time when SLRs were fully manual. They usually had a shutter (T) dial, and the lens had an (A)perture ring.
A mode dial was needed as soon as automatic exposure was introduced. Cameras could have M, Av, Tv, and P modes.
P obviously controlled A and T.
DSLRs added (automatic) control over (S)ensitivity, but the mode dials on the DSLRs still reflect the modes from film SLRs. ISO (S) is treated different from A and T.
In my opinion, this gives a bad user interface:
P: should control everything (A, T, S), instead S might be set to a fixed value
M: should be manual, instead S might be automatic.
Av and Tv should preset only one value (A or T), instead S might be preset or not
Instead of four modes (M A T P), a DSLR should have 3^2 = 8 modes: M A T S AT AS TS P
If I want to have these 8 modes on a mode dial, is a very different question.
When I just googled what the 'v' in Av an Tv stands for, I came across the Pentax K5. Apparently the Pentax people think like I do. this camera has 6 modes on the dial: M, TA, A, T, S, P
What do you think? Do you like the 'logic' in Canon's mode dials?