Past employers of mine have given away redundant kit to employees without wiping the disks.
My wife's former employer would sell old PCs to staff with the disks wiped and a clean new install of the original OS. We still have a (working) Windows 2000 Pro machine from this source.
My current employer requires all kit to be returned to a central point for secure disposal. Some of our work is for the UK Government, where the rules are very strict. We all know that deleting a file does not erase it but merely marks the disk space it occupied as available. However, even erased files can be recovered using special techniques. There is software approved by the government for erasing disks which have held their data, which involves multiple overwrites of the whole disk. Otherwise the drives have to be physically destroyed with the platters being broken into tiny fragments.
Most of us do not go to these lengths though. The other thing is, people use kit until it stops working. How do you erase it then? It is not to hard to take a drive out of a non-functioning PC and format it, but not so easy if the drive itself has died. It depends on the data that is on it to a large extent.
I have a quote of £85 + VAT (20% tax) for the certified destruction of up to 10 items. How many of you would pay that for peace of mind?