I use my bike (gt avalanche 1.0) to commute to work. Now it's autumn and so rain can happen just every day.
So the questions are: is leaving my bike outside harmful? Should take the seatpost with me or better leave it in place?
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It's no problem, but there are a few things to consider:
If you ride in the rain, then keep the bike in a shed on a damp day, it won't dry out properly for hours anyway, so the actual outside storage isn't much extra punishment on top of the ride (assuming of course that it's wet all day)
Avoid taking the seatpost with you if you expect rain. Rain will get into the inside of the frame, and pool down in the bottom bracket. This will soon lead to a very squeaky bottom bracket.
I would refine the comment above: the drivetrain should be both well lubed and dry (no water and no oil). An oily or greasy chain is a magnet for dirt and wears much faster.
It's also probably worth trying to see if you can find a way to store your bike indoors at the office. One day in the rain won't hurt your bike too bad, but continuous storage outdoors will add up.
Be sure to clean that drivetrain often as you ride in the wet.
I'd say that water falling on a bike for a short period of time, will not cause any harm, if by short we consider less than an hour. The water staying in the bike though may cause corrosion. That is, it takes time for corrosion to occur.
That's why you can perfectly wash your bike, provided that you shake the excess water off and let it dry before you store it in an enclosed place.
If you "park" the bike outdoors I would consider some factors like time of the rain event. I would say that it is worse for your bike if it rains on it in the morning and it stays stationary and wet for the rest of the day, giving enough time for corrosion to occur. If the bike gets soaked but is ridden shortly after, vibration from riding and circulating air will shake water off and in some cases almost dry it, so a rain in the afternoon, just before you get out of the office will do almost no harm.
Another factor is whether splash will drag dirt and other foreign matter onto the bike. That's why it helps if you lock the bike in a clean, dirt free location. These concerns, however, are no different than riding on wet streets/trails. A properly lubricated bike should survive with no problems.
As with water, a little or even a lot of mud will cause no harm if cleaned off soon enough.
I ride mostly XC in a tropical country, here it rains all year round (some seasons more than others) so it is highly usual to get home with a muddy bike.I just wash it as soon as possible, shake it a little and let it rest indoors. I have absolutely no problems caused by water or mud.
An aluminium frame will mostly have no problem at all with water. An steel frame should be kept with a good paint job, covering any metal-exposing scratches with any suitable paint or even nail polish. I also reccomend sealing vent holes (Near the ends of stays, they are there to let out welding fumes during fabrication). This can be done with hotglue, gasket silicone or electrical tape. I would seal or cover the seatpost collar and headset bearing if I suspect water can easily get in.
Sealed bearing hubs and bottom brackets are almost immune to gentle rain. Others have proper "dust caps" tha makes them well suited for rainy environments as long as mudy riding is not involved. For non sealed bearings with no adequate gaskets, repacking them frequently with fresh grease is recommended. I would suggest once every two months, depending on the particular conditions.
For the drivetrain, specially non covered chain with derailleurs, frequent cleaning with proper lubrication is the only way to go. I rely on visual inspection to determine whether it is necessary to clean "now!". When there is obvious grit accumulation on cogsets or the chain is covered with a paste-like oil-dirt mixture, it is time. And old toothbrush and 20 minutes of patience is enough. Lube after drying.
All of these tips are my regular preventive maintenance for mountain bikes ridden on dirt trails. For commuting on good city steets same measures apply, only not so frequent.