It's not even anywhere close to a dealbreaker. In fact, I prefer a phone without the expansion slot. I had it on my Treo 600 and 755p, but I never used it and got annoyed with the card accidentally springing out if I sat down wrong with it in my pocket.
I have around 12GB total in all my music, which is 3000 songs or roughly 150 hours... If I ever need to listen to music for 6 days nonstop without sleeping, you can go ahead and shoot me in the head. And that STILL leaves me 4GB left over on my Pre 2. Of course, I prefer to walk around without headphones on 24 hours a day, to hear bikes and skateboards coming up behind me, cars driving near me, birds and squirrels chattering, the wind blowing through leaves, etc. My 8GB Pre- had 6GB free when I upgraded to the Pre2.
If you have a problem fitting all your photos, all your movies and all your songs onto 16GB, maybe you should look at what you put on your device and why. Do you need over two thousand 8 megapixel photos on a device that can only display 1/10 megapixels? Maybe you can resize some of those photos if you need that many carried around on your phone. Do you need to put multiple 720p two hour movies on a phone that can only show 320p? If you do the conversion from DivX/MP4/whatever to the movie format Pres use and scale the movie properly, I'm sure you could fit more than 12 full length movies on there.
As far as the people arguing that businesses would want that because they could buy the cheaper version then upgrade, you are mostly clueless. A) Businesses don't give a crap if they give a phone to their workers and they can't fit all of their music and movies on there... the phone is for business use (email, documents, etc that can be used on a phone or needs to be temporarily transferred) B) Businesses would rather not deal with upgrades and addons after the fact. C) Businesses would rather not deal with detachable parts that can be lost or broken.