| I don't think it's a mistake to use your index finger to hit the first note on the d string with your index after having hit the last note with your index finger as well on the g string as you are still muting the string, playing the next note perfectly audible, and setting yourself up better for the descending pattern/scale/anything.
The other thing I would like to bring up, whether it be a slang term or not, is how would you describe a downward sweeping technique where you are taught to use one finger to sweep between 2-4 strings if you are playing something descending from the g to a lower string? It would seem very odd that this technique would be taught if it were not efficient.
My two cents on this is that whether or not it is a proper technique, it has been accepted by many bassist even most pro players as you say because of the efficiency of doing so, and should be regarded as a proper way of playing the bass, which really all it comes down to, or rather all it should come down to is how you feel comfortable playing the bass. If you can do it while completely alternating your fingers, or if you save yourself the hassle of always switching fingers for relief, it's entirely up to you.
For example, there is one song that I play with my band, where, during an acoustic part I do a kind of walking bass line, which then rests on C for a whole note, followed by 3 16th notes octaves of C which I pick with my middle finger all three times as to replicate the same note and maintain an even tone. | 
