Why I use old hardware
For my laptop(s) I tend to go with the best I 
can afford for multi-purpose development (because everything I do is HPC oriented or virtualized with a need for SMP vms)
but I still have a Dell e6400 latitude that provides directory, dns and monitoring services for the in-house testbed. When it dies I have a celeron road warrior from 
WalMart ready to take it's place. Fit purpose to function to hardware.