Why wait until your system slows down before defragging. You wouldn't
wait to run a virus removal until your PC was infected would you?
You're comparing apples and oranges. With a virus you're either
infected or not, and when you are it can cause problems immediately, so
preventing it from ever happening is important. On the other hand,
fragmentation is a slow thing that may take months before it is
noticeable, if it ever is. So with viruses it's a black and white
good/bad situation, while with fragmentation there is a lot of tolerable
gray area. Also, considering that defragmentation itself takes time and
system resources, it may not be worth it for some people where the time
they spend defragmenting doesn't make up for the time they gain by
having a defragmented drive. This is even the case if the
defragmentation only causes brief spikes in hard drive use, because that
can still delay other software that may also need the hard drive at that
time.
[snip]
Third party software does provide the lesser resource usage, no-impact
approach that makes regular defragmenting plausible and beneficial.
[snip]
I don't see how you can have a "no-impact" approach if it uses any
system resources at all, which it must. Even if you have it run while
the machine is idle, it may affect other resources that also run when
the machine is idle or the user may return while the defrag is still in
process. I know some people who have very little free memory on their
computer, so having anything extra running slows them down. "Low
impact" may be possible, but not "no-impact."
The reason to wait until the system slows down is so that, if you are
one of those people where the impact is negligible for long periods and
the delays caused by a defragmenter would be worse than the
fragmentation, then it makes sense to schedule it when it becomes
noticeable and you will not be using the computer at the time. This
strategy is especially useful for people who don't like to keep their
computer on all the time.
Obviously that is not true for all people, but it is true for some
people.