I've heard horror stories of folks zapping a computer using these. With a passthrough battery, the battery acts as a buffer and protects the computer. Usage is almost exactly the same, just unplug it every once and a while to avoid the batt gaining a short term memory.
Fortunately, lithium batteries don't develop a memory so that's not a concern for us.
You should still unplug. It can shorten the lifespan of a lithium battery to perpetually sit near the top of the voltage range. They're happier if they're drained a bit and then recharged at that point. I've read that 3.92 volts theoretically reduces voltage-related stresses to nothing, so that would actually be the optimal voltage for the battery. None of us, of course, can perpetually keep ours there.
I tend to unlink my battery passthrough in the morning, cadge a charge in the afternoon, unlink and charge again in the evening. On a 1,300 mAh battery, that's still overkill and shortens its lifespan, but not as badly as leaving it plugged in all the time.