I never said it was for everyone, but I suspect people magnify the expected issues a lot.

Everyone knows I won't be here on wednesdays, so we try to arrange things so I'm not a bottleneck during that time. If some really important deadline shows up, I'll just work the one wednesday and take thursday off instead, it's not a big deal. I haven't needed to yet though.

Besides, assuming you're not on a really tiny team, if many parts of your project hinge on one person with no available backup, you have bigger problems related to project management.