The moment the mobo touches the case pegs it's at the same potential as the case, hence if you're also touching the case the three of you are all at the same potential.
Static is interesting in that it builds up as a capacitance, and even more interestingly you don't have to put the electrons back where you got them. Hence, the earth in general (ground, heh) is a great way to get rid of your static charge, it has far more capacitance then you do and can suck those electrons up without an issue.
The case, not so much, so you could in theory just be sharing your charge with the case and then sharing it with the mobo. If the mobo somehow got grounded while it still had that charge you could have an issue.
Really though, it's only the last two computers i've done plugged in, and only the first three that i used a wrist strap (to the unplugged case) on. The other couple dozen (more? I lost count) i've just touched the case often, or held onto it.