Why do you lose all of our comping work if you record another take on top of an already comped together track? Inevitably after listening to a vocal for a couple days, I'm going to want to go back and record another take or two where I concentrate on the trouble spots. If I record the to the same track as the original vocal, all the work I put in to comping is erased. Is there a reason someone might prefer it that way I'm just not seeing?

The reason must be that it is setting the new track as the one you want to listen to. Otherwise you would be listening to the prior takes instead, right?
 
It would be kinda confusing to not have most recent recording win. If you don't then no new track would get played on playback until you went through the comping exercise.
 
You could work around it by just recording it on a new track and dragging that clip into a new lane on the old track. I'm not sure this is the best solution, but it would work.
 
Alternately, you could bounce to tracks the clip before recording the new take. This would give you a copy of it forever but it could get messy trying to keep track of them all.
 
 

BTW, if you add a lane by setting a current lane to clip and clicking the +, you can switch record method to sound on sound and record to only that new lane. This does not over-write previous comping but lies your new lane on top of the old.

This is one reason Sonar included the "flatten comp" feature. After you piece together your comp, flatten it and it appears in a new take lane as a new clip. Now it behaves essentially as a single take that you can use for any for any subsequent comp. It is also helpful if you have more than one comp that you like. Flatten both versions and the you can decide later which one works best.

Nice one Neirbod.

Also remember that after creating the flattened clip, in many cases you can just punch over the problem spot.