Would depend on the overall effect you're looking for, don't you
think?

-- JC

Depends on your subject, background, and look you're going for.  If you
photograph a dark haired subject against a dark background some sort of
separation is needed.
If you photograph someone with thinning hair it tends to make them look
balder than they are, and even sweaty or shiny headed.  Any time the
hairlight is too strong, it can wash out all detail and looks like the
photographer doesn't know what they're doing.
On the other hand, a good hairlight adds dimension to a portrait, and makes
the hair look healthy, and makes your subject look more lively.
So I guess the answer is it depends.

I worked for many years without using a hairlight.  Depends on what you
want the final result to look like.

Hair is the face's frame.   It is the most easily adjustable feature we
have.  If people go to the trouble of selecting a style that suits them then
it is worth the effort to show it.

Also, it is helpful to sculpt the shape of the head, to give some separation
from the background by having a counterpoint light from the key light.  I
think a hair light is the second light a photog should use after getting a
decent mainlight that doesn't require a fill, or if you are going for
Humphrey Bogart look, a stark film noir look of a specular key and a hard
kick light (a hair light thats aimed at the side of the head positioned much
lower, usually opposite the key light)

consider the nicest outdoor light, that delicious twilight where peoples
eyes seem to sparkle and their hair glows like angels.

this reply is echoed to the z-prophoto mailing list at yahoogroups.com

Depends on your intentions.

Basically there is not much to add to Zeitgeists answer,
just that I always prefer a controlled soft light as
hair light, mine is a backlight bank that is 15 x 200 cm
wide with front diffusor and louvers, fired by a Hensel
130 cm longlight on a 3200 Ws powerpack.

If you use a single (compact) head, you will likely
return with a highlight hole burnt into your subjects
head.