I still haven't parted with some of my junkers (like my Red G2 Snarl who's robot mode leg is snapped off).
Anything that I got when I was a kid and unable to fund my collection myself I have a hard time parting with. These were the toys I played with rather than my collected pieces. I sold off all my alternators recently without batting an eyelid, they were just collected pieces to me, whereas if you asked me to sell off my Laser Rod Optimus Prime (which will never, ever happen), I'd just shrug it off.
I could buy another G1 Blades for example (one blade is broken off), and I don't have all of the Defensor parts (in fact I'm missing Groove and use Fireflight instead) but I wouldn't ever sell them off because of the history connected to them. Same again goes for my beast wars toys (which has surprisingly lasted).
However all my Cybertron, Movie and Classics toys are complete and have boxes and instructions for the most part. I could part with any of them.
Strange isn't it. I understand totally about the Ratchet from Santa. I got BW Cheetor (original, not TM or TM2) for my 8th Birthday from my step-dad, and the memory of that always makes me smile when I look at the figure.