Q: why would you voice something in second inversion, like Bma7/F#, for example? does that all have to do with the chords and voice leading around it? also how would you voice that on keys
A: it just sounds a little different. you'll have to try it and see. actually i can give you a really common example of where B/F# really is what you want. actually let's switch it to the key of C for easier explaining, and it's not actually Cmaj7/G, this example is C/G, but nonetheless i think you will be able to see the difference. Take the song 'happy birthday to you' in the key of C. right before the end of the song, on the last "birth". the harmony is probably playing C chord, but i think you'll find that the best place for the bass, at this point in the song, is G. i can name a trillion other examples of popular/classic/traditional songs, over the course of the last 300 years, that have had a 1(chord) over 5(bass) absolutely thrive in this right before the end finalistic 'the song's about to end' spot. you've heard it a million times. try putting C over C there instead .... it doesn't quite fit. it sounds like the beginning of the song, .... right before the end ? ... By the way, to complete the example just you you can hear it clearly, the 'good' way to do it, from the final line "birth" is C/G, "day" is D7, "to" is G7sus4, and "you" is C, preferably with the G note left out, just e c , with low C in the bass. and Fmin/Ab was on "hap"