My view is exactly the opposite.
This notion stems from the technical support people at many of the larger
OEMs. Their solution to almost any problem they don't quickly know the
answer to is "reformat and reinstall." That's the perfect solution for them.
It gets you off the phone quickly, it almost always works, and it doesn't
require them to do any real troubleshooting (a skill that most of them
obviously don't possess in any great degree).
No, there is often a LIABILITY issue that your mindset fails to address.
If a customer brings you a compromised machine, while any number of us
could 'clean' it well enough for our own use, if you returned a machine
to a customer and they could prove it wasn't cleaned and they experience
some real loss, then you can be liable.
When money is concerned, charging the customer, few cleaning events take
less than 1 hour of time, in fact, even a full scan of a typical home
users machine can take several hours and then you most likely didn't get
it all and have to go back and edit the registry, delete folders, etc..
Sometimes it's better for the client to do a wipe/reinstall.
But it leaves you with all the work and all the problems. You have to
restore all your data backups, you have to reinstall all your programs, you
have to reinstall all the Windows and application updates,you have to locate
and install all the needed drivers for your system, you have to recustomize
Windows and all your apps to work the way you're comfortable with.
In most cases you can use the restore CD that was shipped with the
computer, as most people buy a package from Dell or other large vendor.
In the case of a Dell you can install all of the drivers at one time,
without rebooting, and be up and running in under 2 hours (from a fresh
install), and that can also include shipped applications.
Besides all those things being time-consuming and troublesome, you may have
trouble with some of them: can you find all your application CDs? Can you
find all the needed installation codes? Do you have data backups to restore?
Do you even remember all the customizations and tweaks you may have
installed to make everything work the way you like? Occasionally there are
problems that are so difficult to solve that Windows should be reinstalled
cleanly. But they are few and far between; reinstallation should not be a
substitute for troubleshooting; it should be a last resort, to be done only
after all other attempts at troubleshooting by a qualified person have
failed.
Noone said the machine would be restored back to the before compromised
state, but, as you can't tell anyone that the machine is 100% clean,
you've got risks with a 'clean' method vs a wipe/reinstall.
While you might think that you can 'clean' a machine, I'm 100% certain
that you can not clean it of unknown malware 100% of the time.
And perhaps most important, if your problems stem from something you've done
wrong, or safeguards you haven't put in place (and in the great majority of
cases, it's one or both of those), by clean installing, you never find out
what you did wrong. After clean installing, you're very likely to soon
repeat your errors, and find yourself with the same problems.
Most people soon learn, if you tell them what was wrong based on the
malware actions, how to at least increase their chance to survive, those
that don't will be back. The more times they compromise their machine
and have to pay for it (time, money, pain) the more likely they are to
not do it again.
What it comes down too is that sometimes the cost is important,
sometimes the data is important, sometimes the customer is willing to
accept "as clean as can be at this time".