Take care - ther be dragons!
After I bought my 20D, I soon found that swapping lenses all the time was a complete pain (just as you have found, Postcardcv), yet after the substantial increase in speed, flexibility, and above all picture quality the SLR brought over the P&S cameras, I wasn't really happy to go back to the 4500s or even the A95.
So I bought a second 20D. I wouldn't go back to one camera again in a blue fit. It is a mile better than buggerising about getting dust into the system and missing good shots. Yep: go for it.
One problem. Especially since I added a 500 to my 100-400, I often want a third body. Luckily, I haven't figured out a sensible way to carry all that gear yet, so I've restrained myself.
The (only mildly insane) two-camera setup works like this: big lens on one body (usually the 500, sometimes the 100-400 if I need close focus more than I need reach and f4), this one held in my hand or on the strap. The second body lives in a Lowepro Specialist 85 bag (superb bit of kit) that has a combo waist belt and shoulder strap, which sounds weird but works really, really well, and is easier to carry a decent weight in than a belt pack, a shoulder bag, or a back pack. On the second body I'll have the 10-22, the 18-55, or the 60mm macro, plus the other two smallish lenses, a 1.4TC, a 12mm close-up tube, and various odds and ends such as lens cleaning gear and an angle finder.
I can walk around comfortably, reach any bit of gear easily enough, and be ready at short notice for whatever comes along - be that bird or stunning landscape or tiny wildflower.
There are still improvements to be made, nevertheless.
First up, I want to figure out a way to carry the 100-400 without buggerising about reversing the lens hood. (I don't like not having it handy, no doubt for the same reason you don't like not having your 100-300 around.)
Second, though I can hand-hold the 500 well enough, and am finding that hand-holding is the way to go for most passerines, there is nothing quite like that absolute clarity you get by putting the big 500 on a tripod - yes, even when the light is perfect. So I want to figure a way to cary a tripod easily, so that it doesn't get in the way and yet I can get at it readily when required. Some kind of backstrap harness, I guess.
Third, I need to take Charles Atlas lessons so that I can carry all that gear around without feeling like I've escaped from a train wreck afterwards, and maybe wear snowshoes so that my feet stop sinking six inches into the ground with every step!
I'd dearly love to have a third body so as to avoid constantly swapping between macro and wide-angle lenses (I hate dust!), but I just can't see how that is going to be practical.
The long and the short of it, Postie, is yes, a second body is a very real improvement. Go fo it!