Thank you for spelling out the argument.  Now we can examine it.

    By LGPLing gPhoto2, we allow for non-free software to use gPhoto2 and
    the camera libraries. We push into other platforms, specifically
    GNU/Hurd, Windows, Macintosh, BeOS, *BSD, Solaris, etc...

We could do this if we release part of gPhoto under the LGPL, but we
could do this just the same if we release it under the GPL.  We will
certainly make gPhoto run on GNU/Hurd, and it is likely that people
would also port it to some of those systems.

							      With a large
    user-base and application support, we become recognized as the
    cross-platform solution for digital camera drivers (write 1 driver, run
    on all platforms).

That is the most favorable possible outcome, but it isn't likely.
You're envisioning that we will win a competition against clever
competitors.

It is common in business to enter a competition bet a great deal on
the long shot of winning it.  Sometimes you win, more often you don't.

GNU is not a business, and we should not take big risks hoping for the
big payoff.  We should aim for less risky plans.  The main point is to
provide people with a way to do a job.  If it happens that our
software is so good it becomes a de-facto standard, that is a nice
bonus.

		       Camera manufacturers take notice of this and begin to
    write or sponsor development of gPhoto2 drivers for their cameras

This step of the argument seems solid.  If we win the competition,
camera manufacturers will strongly want gPhoto to work with their
cameras.

Even if we do not win, even if gPhoto does not become the dominant
interface to cameras, gPhoto may still be sufficiently popular that
manufacturers will care about supporting it.  Since the other software
alternatives do not support GNU systems, and since GNU systems are
becoming fairly popular, I think camera manufacturers WILL care about
gPhoto support to some extent.  I think this will happen no matter
which license we use.

    (source-code provided).

This does not follow.  The practical reasons that would make the
manufacturers want gPhoto support would not make them release source
code.

At this point, if gPhoto is GPL-covered and manufacturers must release
source code to obtain gPhoto support, many of them will release source
code.  But without the push of the GPL, they will tend not to release
source code.