Ah, biomorphs... well, while it is certainly possible to fight, and fight well, without them, biomorphs add a lot of flexibility to your swarm. You just pick 'em, and add up the points. If you can, try to beat the WS and I of your main opponent by 1. Extended Carapace is only good on the bigger creatures. Toxin Sacs can be good, but it depends on what you want the creature to do. Wings are good on larger bugs, but for the smaller ones you're ususally better off with Gargoyles. Leaping is okay. Flesh Hooks are like grenades, so if you plan for a lot of cover and walls, take 'em. Implant Attack is only against armies with a lot of 2-wound creatures (Thousand Sons?). Enhanced Senses is a must if you have a gun. Bio-Plasma is cool, but expensive.
Generally, find a purpose for a species, and take only the biomorphs related to it.