Sitting up in the pointy end we have absolutely no reason to lie about why we're delayed. If we get put in the penalty box, it's not because we have a mechanical problem that we just don't want to tell the pax...we're pretty sure they'd notice when the mechanics start taking parts of the airplane off and subsequently climb on board to fill out the logbook. If we say it's wx/ATC, it's just that. If we don't' say anything else for the next 40 minutes, it's because we don't know anything else. When we're given an EDCT or wheels-in-the-well time, they're probably not going to say anything more to us (except shut the f-ck up if we try to talk...especially in New York) until that time passes. While the gate agents and airlines on whole may not be nicest to their passengers, as pilots, we have no reason to lie about a delay. If it is mx, we probably won't be going into detail over the PA as to what the problem/fix was (mostly because most people don't care that much), but if it's a mx problem and nothing else, we'll say that. We don't use "weather" as a blanket excuse. As Jesse said, the nexrad image doesn't show everything. Unfavorable winds or low ceilings (even without storms that show up on radar) at departure or destination will slow everything down. Storms en-route, something that everyone forgets about will shut down entire departure/arrival gates, or make us decide to park it for a while until a system passes. If it's raining in Atlanta, all of the NY airports are going to be stopped because they run out of room to park all the ground-stopped outbounds and they start to get in the way of the folks that can actually leave. If it rained in Boston three hours ago and it's cleared up now, they're probably still going to be delayed catching up on all the arrivals that were pushed back during their flow program. The current weather this minute at the arrival airport isn't the only weather that can affect the go/no go of a flight.
Sorry if this has turned into a rant, but I was accused of lying about a delay three times last week and when I asked "what would the CA or I have to gain from lying to you about why we're sitting here picking our noses, next to seven other planes that haven't moved in the last 20mintues?" no one could seem to answer. I'll be the first to admit that gate agents can be schmucks sometimes, but they get yelled at all day...it's hard not to be. No one has anything to gain from lying, though, least of all the pilots. When you're delayed, so are we.