I was reading the bug codex and it occured to me, they dont really have any viral weapons. Having just been very sick, I got the feeling that the Tyranids could be much more efficient if they could create some virus' that would just kill us all. Sure, they have all those spores and stuff. But those spores dont kill you fast enough.
They need something like Meningacockle (me spell good) or a super flu or something. Since they break everything down to a cellular level, it stands to reason that they could convert some of those cells to Viral-Nids that could kill pretty much anything organic they come into contact with. And considering the speed with which they take planets, a little decompasition of the defenders would be much better than expending all their bug-power.
Also, the virus wouldnt stop evolving. It would be ever-changing just like the Tyranids.
An image which comes to mind is the episode of Itchy and Scratchy where they're taking off the Fantasia theme, and Itchy (the cat I think) chops up Scratchy into peices so fine that when he inhales them, they start hacking his lungs to pieces and he dies from the inside out.
In summary, if they had viral weapons, then they wouldnt need as many bugs to do the fighting for them. They could just concentrate on making "gatherer" Nids for the planets they conquer. Sure, they'd still need warriors and such for the people who somehow escaped the germ warfare, but they wouldnt need as many.
Thoughts people?