Its a lot easier to move away from an animal if it is too close than it is to try and get closer if it is too far away
... unless of course it is chasing you
seriously though.. depending on terraine and the animal sometimes you cant get far enough away because there are too many things in the way at a greater distance or the animal just desides to come visit and you cant get far enough away.... I've had that problem several times with the 100-400 and whales... when they are in the mood they can be really curious and when they do deside to come close sometimes you find yourself taking a pic of the blowspout or a partial fin cause it is just too close. You also have to worry about atmosphere (fog, mist, rain, heat in dry areas)... on these days you want to be as close as you can be to cut down in its effects. I guess that is where a second body comes in when you just dont have the time to switch lenses or where you are in an eviron where you dont want to change lenses unless absolutely nessessary.
With wildlife though it is really about finding a happy comprimise unless you are able to carry a lot of large expensive gear. The size of a 600 prime alone is prohibitive if you are going on a hike of any kind or multi day outing. The 100-400, for me, is that happy medium and I rarely find the need to switch lenses or a better way to put it is that I can more often than not find a way to use the 100-400 effectively.
I'm getting a second body within a few weeks.. the goal is to have the 100-400 on one body (7D or another 50D... havent decided yet) and the 24-105 on the other (50D)... that should cover every situation possible... in my pocket I will carry the 100 macro just in case and swap it with the 24-105 the odd time I need it.... for bugs and flowers and such....
Canon 7D + 50D + EF 100-400mm f/4.5-5.6L IS USM + EF 24-105mm f/4L IS USM + EF 100mm f/2.8L Macro IS USM
Pelican // Black Rapid // Think Tank // Manfrotto // Garmin
Reflections On Canadian Wildlife