Perhaps off-topic, but I'm putting together a trip for October based on a conference in Barcelona. I think one full day between arrival and conference start is justified, given that you have to allow for delays, jet-lag, transfers, all the "friction" of modern travel.
If all goes well, it's a day of sight-seeing. If not, well we've all been there. You don't want to miss any of the conference because you're groggy or not actually present, or you are stressed over missing luggage or whatever.
Contrariwise, I'm hard pressed to justify time after the convention. The thing's over, and unless you can run two of these things together, there's no reason to hang around.
Having said that, there's always the possibility of arranging things so that the most direct route home is not the way actually taken. Taking the first flight out after the closing address may leave you at the mercy of cabdrivers or whatever. You may need to wait a day before the next connection. And if there is a difference in airfare, then why not take a longer and more indirect route home?
If the fastest trip home gets you back on a Friday lunchtime, you aren't going to be much good for work on Friday arvo. Might as well have a day or so extra on the way home, aiming for a bright and rested Monday start.