You do not need a big knife. I like my Buck 103 and my custom. The Buck 103 is a 4" blade. I am not certain what the custom is. It was made by George Anderson out of Utah in the mid '80s for me for the princely sum of $45.
It is not vital but it does make it easier to get all that stuff out. If you aren't packing it out a great distance or if, like us, you can use an ATV to bring it right to the truck and then from the truck home and to the processor then there is no need to go to the trouble of quartering and boning. Like you said, let the experts do the work.
I have never seen a whitetail that didn't open like a zipper when opening the breast bone. It is so easy and it gives such greater access to get out the windpipe and the heart and such that I can't see not doing it.
If you are quartering don't you have to do it anyway?