The interesting aspect that a lot of people are overlooking is that, for a theoretical attack within certain timeframes, this difference can be make-it or break it! Imagine a collision attack that takes about a 1 year with 64bit serial numbers, so with 63bit serial number it should take about half, at 6 months. The average certificate is issued for about 1 year, so being able to mount a collision attack that took 1 y…
Why do you assume that an attack would take 1 year, and not (e.g.) a billion years? A factor of two is only interesting if the number you're dividing was interesting in the first place.
imagine is hardly assuming. But it doesn't have to be exactly 1 year, any attack that takes longer but less than 2x average certificate lifetime with 64bit serial numbers (useless) becomes practical on 63bit serial numbers (useful, for a strange meaning of useful).