My original generic case had a removable motherboard tray, I still ended up disconnecting every cable, and removing the motherboard from the tray anyway.
I prefer to have the motherboard on a antistat material with a foam backing when I'm applying the forces necessary to put a heatsink on. I'd rather some support behind the socket then standoffs placed around it, watching the motherboard bend wasn't something I liked seeing.
just think how often you really need to have access to your motherboard outside the case and that will answer your questions.