Modern solid state memory cards are exceptionally reliable, but not totally infallible. Currently I own 4 32GB cards, and a pair of 16GB cards for my cameras, but in general only a single card gets used for day to day shooting. The extra cards only get pulled out during long event shooting. If I'm out photographing a series of games then I will devote a single card to a given section, and swap at the end of it and ready another card for the next section. This breaks things up nicely when I get home and want to quickly organize everything into sub groups based on the game.
So for a double header I will do the first half of the first game on one card, the second half on another, the intermission show on one of the small cards, and split the second game on the other two large ones. Second 16 GB gets carried around in a case tucked in my pocket for emergency spare.
For longer events, such as weekend tournaments, where there are more individual events than what I have cards for, then I will cycle them in the field. The laptop comes along and cards get downloaded to the laptop, backed up to the external drive, and then scrubbed while I'm shooting the next game. Helpful if I can have someone tending the laptop to handle the upload and scrubbing so I'm not rushed between games. I don't even sort them into groups in lightroom, just keyword on upload with an identifier for each segment, and then filter by that later.
Now that I'm planning to add a second body to the mix for the new year I think things might get a little different in workflow for longer events. I will likely time sync both cameras, and then just shoot straight though on a given card till it is nearly full, then keep careful notes on times. (Photo of your boot to give a visual breakup between sections was an idea I picked up somewhere.)
But for other things, if I'm just going out and taking a small handful of photos, then I don't bother with thinking about other cards at all, beyond making sure I have a spare kicking around in the gear bag for if I get an error.
I'm a firm believer that one should always have a spare card with them, even if you have no intentions to use it. The idea of holding all your eggs in that one very reliable basket rather than trying to juggle stuff is all well and good, but if that reliable basket Does break... Well then you're smacked twice as hard, as you have not only lost everything you've already taken, but you then can't take anything more after the fact. Having spare cards has saved a bunch of photos a few times due to someone accidentally formatting the wrong card. We were able to stash the 'deleted' card away, and run recovery software on it after the fact.
Memory is cheap, so the long of it is to not sell yourself short by not having more than you need on hand.
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500
Flickr: Real-Luckless