Why do scientists consider the loss of biodiversity, that can affect
genetic variation, to be such an important issue?

Genetic variation is what allows us as a species (or any species) to
survive during changing conditions.  The loss of diversity seen in
bottlenecked populations can be devestating - primarily when you look
at the succeptibility to disease.  Genetic variation in the genes that
control immune response is key to the ability of an organism to fight
off disease.

Jared Diamond is a great author to read on the topic.
I would recommend The Third Chimpanzee.
http://www.amazon.com/exec/obidos/ASIN/0060984031/002-9254155-8266442

Hmmm, are we not mixing two concepts here?  Sure genetic variation is
vital toward the long-term benefit of a species.  But genetic
diversity across species is another thing, unless you are suggesting
that horizontal gene transfer to H. sapiens from some of those other
species is taking place?

I think the answer is that diversity (and the broad range of species)
in part defines the extant food chain upon which we (presently) depend
for survival, so don't mess with that without being sure of exactly
what the consequences will be (which the present state of the art of
biological sciences is unable to predict).

I am sitting in the midst (and the mists) of the Costa Rican rain
forest as I write this comment...

Biodiversity acually gives rise to the increaesd number of species tht
have differential adaptations because of the genetic variation it
produces .Now consider an extinction effect generated by some sort of
catastophe like ice age.
           Due to such events most of the species in a given
environment will die off,but some will survive that have genes that
help them to adapt to such conditions!Larger is the biodiversity in a
particular area larger will be the number of surviving species.
        An example of this event is extinction of the
dinosaurs.Assuming that the great lizrds died of some change in the
environment that they could not adapt to,the mammals must have been
able to survive that event because they had the genes to stay alive in
the changed conditions.
             Even in humans not every one is able to remane immune to diseases
such as the SARS virus that had struck recently,Only some suvive and
the transmit their immulogical advantage to the next generation as it
is genetic in nature.Thus diversity among a single specie is also very
important.
            Actually even behavioral diversity also plays an impotant
role in the survival of the developed organisms like
mammals.Especially in humans,some races or kingdoms which could not
adapt to the changed ideas,technologies of the new world vanished e.g.
the mayans,the incas or similar races,because they assumes them to be
'unholy' and religiously unacceptable.
         Thus diversity is essential for the survival of species to
face severe conditions like new diseases which require an effective
antibody response or severe climatic changes.