I'm not advocating ewither, but the slight difference in whiteness is
moot with digital, unless you are using a fixed WB preset.  A filter
that is slightly red will actually bring the mnatural WB of most
digitals a little closer to white, anyway, as they tend to capture
everything greenish-cyan at the RAW level.

I strongly disagree that 1) that it's the dealer who wants to scare you,
and 2) that there is any image degradation, at least with
reasonable quality filters.

I use Hoya super coated UV filters on my lenses.  I used to
use tiffen.  I have never seen an issue with degradation of
image quality, except for the following:  Some 35mm lenses
have a pretty flat front element.  This can result in a reflection
between the front lens element and the filter (any filter).
I have found reflections in images with highly transmitting filters.
(I have not seen the problem with circular polarizers).
The Hoya super coated filters do not have this problem, and their
optical quality is high.

The advantage of a filter is that it protects the front lens element.
For example, when you work outdoors you will get dust on the optical
surface then humidity/water spray/rain moistens the dust and
glues it to the optical surface.   Water and/or humidity
partially dissolves the minerals/chemicals in the dust, which
if not completely removed, will later precipitate onto the
optical surface as the surface dries, cementing the dust particle
to the surface.  You can clean it carefully, but over
time the optical surface and coatings degrade.  One can simply
throw away the filter an put on a new one when you get to that
point.  High humidity environments will do the same.

Examples: Hiking in the mountains: dust gets on the camera, you
approach a waterfall and spray wets the dust.  Ocean spray
of salt water is very damaging.  Snowy day: snow wets the lens,
gluing the dust down. etc.

If all your work is in a clean room, you don't need a filter.

While a lens hood protects against some of the dirt problems,
wide angle lenses are more of a problem because the hood
can't provide a tight opening.  My 500 f/4 L IS lens has a great
hood and does a very good job of protecting against dust,
and I do not use a filter on it (they are not made that big).

For my smaller lenses, I use step up rings to bring most lenses
to a common size.  That way I only need a couple of filter sizes.
For example, my largest lens (excluding the 500 mm) uses 77 mm filters.
But most of my lenses use 72 and smaller filters.  So I bought step-up
rings to bring those lenses to 72 mm.  I use 72 and 77 Hoya super
coated filters on my lenses and with digital I only carry 72 and 77
mm circular polarizers.  On my large format system, I also use
72 mm filters, and use Hoya super coated UV filters on them and carry
72 mm circular polarizers and 81A and 81B filters.

Roger
Photos at: http://www.clarkvision.com

Don't you have that backwards?  The UV becomes effective at altitude, the
skylight just adds a little red to counteract the blue of the sky for photos
in open shade, etc.

The light loss from a skylight/UV filter is much less than 1/3 stop,
never mind 2/3.  The main reason portrait 160 is shot at 100 is that it
brings out detail in the dark areas, esp. dark hair and clothing, in the
image while not blowing out the highlights.  Color negative film has
lots of latitude for overexposure.

IAC, in TTL exposure the presence of a filter is, er, transparent,
regardless of the filter light loss.

I shoot 160NC @ 100 without any filter all the time.  When I forget to
add a filter it's the 81A under open shade.  Thankfully the lab can
correct that, and so can RAW import.

Cheers,
Alan