I know I'm very late to this party as it's been 8 years now, but I suppose that doesn't prevent revisiting this discussion. I agree with the OP that a reverse bias condition will exist when only using a shift register to drive a multi digit 7 segment display. See the attached image. Let's say we want to light just the DP of the first digit. We enable digit 1 with a high (red), all other digits are disabled with a low (green). Now to light the DP, we have to set the segment cathode to low and set all other segment cathodes high. The blue path indicates current flow to light the DP. But also note the yellow path. We have created a reverse bias condition. The yellow path is just one example, but it exists between all other off cathodes and their corresponding off digit anodes. This could be a problem as I've seen some multi-segment display data sheets that advise against reverse biasing the leds as that can reduce its lifetime.
