For me, the varnish is yet another thing to spend money on. First the expensive model, then the primer (which goes fast), then the paints and other supplies. For the best finish you should use multiple coats of varnish as well, which can make spraycans die quickly. For regular rank and file guys it just isn't worth it to me.
I also like being able to go back and work on old models, or fix things I didn't have time for or notice when I originally painted them. A lot of my first models were pretty bad but I was able to go back and bring them to tabletop quality because I never varnished them.
However these are just my personal nitpicks, and there can be absolutely no doubt that if you want to protect your paintjobs, you need to varnish. I recommend Testor's if you can find it - it works very nicely and I don't think I've had a bad can of it yet. I use a coat of gloss and then a coat or two of matte which cancels out the shininess.
I've never heard of that spray, Soth - do you think its sold in the States? A quick google proved fruitless