There are a few reasons that I decided to keep the old backspace in its place:
* I found that the Backspace muscle memory is very strong. Even after I've been using Colemak for almost a year, I still press the right Backspace key from time to time.
* To lessen the frustration during the learning process. A typical scenario is someone making a mistake, trying to correct it by hitting Backspace, type the right letter, oops, the Caps Lock is on, recorrect again, retype again (and repeat the same thing dozens of times because of the strong muscle memory...)
* In a shared computer, say one of the users switched to Colemak, and the other users need to type a short command to switch back to QWERTY. Usually you can get by even without knowing the layout with copy+paste or trial and error. But it would be completely unusuable to someone who's not familiar with the layout without a Backspace in its expected place.
* Makes it easier to correct errors while switching between layouts. e.g. type something, find out you're typing in the wrong layout, switch layout, try to correct it, etc...
* It makes one-handed typing (e.g. when you're talking on the phone) much easier. Error rate with occasional one-handed typing is much higher than touch typing with two hands. Consider the hand movements required to reach from one side of the keyboard to the other. Assuming an 8% error rate for one-handed typing, having two backspaces instead of one can cut down on finger distance by 15% or more.
* Some ergonomic keyboards place the Caps Lock in an uncomfortable position (e.g. TypeMatrix), and some keyboards don't even have a Caps Lock key (Happy Hacking Keyboard and others). On keyboards that don't have a Caps Lock key Colemak would be unusable.
If you still need the Caps Lock, I recommend keeping the right Backspace in its place, and mapping Alt-CapsLock, AltGr-CapsLock or ScrollLock instead. See also.