Thansal

My argument against too many demos are these:
1) Hitscan.
A good sniper can destroy demos, and the demo will have a hard time retaliating so long as the sniper pay attention to charged stickies. This holds true for any class with good hit scan weapons.

2) Defense mindset.
This is only really a problem with pubbers as it can be overcome with proper planing, but most people tend to think of demos as defense, and thus act like it. Instead of pipe jumping directly onto a cap point they will set up sticky traps. Instead of rushing the SG nest, they pick off people as they leave.

#2 really is my main one tbh. It is much like a team with 3 engies, it just lacks offensive momentum.

All that being said, I think that if you have good enough players a full team of mostly anything could do well (well, maybe not spies :P), and I would love to try and reenact a full team of soldiers as I find it hard to believe that decent soldiers would get trounced on repeatedly.

Fish #641

http://www.thefenix.net/TF2demos/Fish/
I believe the one with soldiers and most of the rest is castle, which is the 4th demo. Though the soldiers' rockets were impressive visually, they simply didn't do the damage that a heavy rush did. The heavies may have been slow, but they were like tanks and did a bunch-a-crunch-o' damage.