I'm lucky in that I can hop on the subway quite easily, and I can leave my bike in my office - so if I ride in to work and decide not to ride home for whatever reason, I can get home (and back the next day) easily, and my bike is safe. I'm more of a fair-weather bike commuter, so I won't ride in to work in caca weather, but if the morning weather's ok I will go in despite a dodgy forecast. More often than not, the weather stays fine and I can ride home.

Like OakLeaf, my concern with thunderstorms is less the threat of lightening (I'm riding in a built-up area, so except for the ride over the Charles River, I'm not exposed), but high winds, heavy rain, and what they can do to to braking, visibility, and drivers' brains. Wind and visibility are big concerns. My partner was once literally blown off her bike trying to get home in a storm. Thank heavens she wasn't seriously hurt, and didn't fall into traffic, but even a less dramatic gust can push you off-course. Add drivers being even more stressed-out and less likely to see you, and I"m more than willing to T it home rather than risk my neck.