Here is what Sony says about A99 weather sealing

No that would mean waiting to change the lens when outside of the dirty environment. There is nothing that dictates you must change lenses on an SLR camera just because the lenses are removable. A good photographer will pick what lens will be the best for the situation before entering it so he doesn't have to change them. If you need more lens options you carry another body with another lens at the same time. The dust and moisture proof specification is based off of using a sealed lens with a sealed body to create a sealed system. Once the lens is off and it is unsealed, the all bets are off. This specification is for the system, not camera body alone and is generally stated as such.

I think it's pretty obvious to most people that when they buy a weather sealed camera body, that in order for the most protection a sealed lens must also be used. But with that said, cameras collect lots of dust and moisture within the contacts of the switches and buttons, especially when used professionally. The buttons and switches are where failures happen the most due to dust and moisture destroying the contacts and are the main reason why the body has seals. But these seals are not a 100% guarantee, hence why it's rarely fixed under warranty. It's purpose is to help longevity of the camera in abusive environments and guard against partial failure (switch wheel etc) but not complete failure as well as lengthen time between services. Pro's regularly have their gear serviced to have the seals checked and contacts cleaned etc.. no different than having to change the oil in your car. That's one reason why NPS and CPS exist.

If you want to relate to it like a car, having weather sealing is like synthetic oil. it allows you to go for much longer periods with more abuse before needing a change. Mobil 1 full synthetic is good for 15,000 miles. Regular oil is good for 3,000 miles. Both of these number are reduced in abusive conditions. No matter what you use though you still require service. For a camera to last it needs to be serviced too. Weather sealed or not. This is service called a CLA (clean, lube, adjust). How abusive you are vs. the build quality of your gear will determine how often you should get a CLA done.