Environmental sealing is an excellent feature on a DSLR camera. Of course, both lens and camera (and battery grip, if attached) need to be sealed, else you will be taking some risks that the lens or grip will let in water/dust/sand/mold.
My Olympus E1 has been on some adventures including:
- lots of rain
-some now and sleet
-a couple drops into snow
-dust, dust and more dust
-uncountable splashes by fresh water, muddy water, salt water
-multiple times has been dropped in the surf
-once was dropped in the surf and misplaced for about 10 seconds until the waves receded enough to get my hands on it
After all this it's still going strong after many years (as are the lenses) so there is certainly a strong argument for environmental sealing, even if it's just to keep the dust and mold at bay.
In saying that, bodies can be replaced every few years, but quality lenses are long lasting purchases, but if I had the choice I would try and go for a camera with environmental sealing, and choose lenses that offered the same degree of sealing.
edit: I forgot to mention, I also dropped the body with 50-200 lens attached, right onto the front tip of the focus ring ... the lens was wearing a UV filter at the time, which took a nice little knock, but otherwise the lens survived and still working fine.
Brad

