Spiders are not weak. Recluse are very good skirmishers and one of the best attack-move units. Crab on a spire is a legitimate threat which, if backed with a group of cons repairing it, can hold down a large area by itself. Widow can be devastating if used correctly and early. It emps enemy commanders and the big rushed units that can cause so much havoc.
The most important unit is flea. Almost every freaking game I find myself chanting "we need fleas" and "flea screen, where?"
Let me spell it out.
Your big hordes of artillery, Lances, Impalers, etc are nearly useless IF THEY DONT HAVE VISION. Cloaked fleas provide eyes-on-target which make team artillery twice as effective. Their secondary role is to trip up sneaky enemy crap that can ruin your day. All the cloaked snitch and cloaked reaver tricks are stopped by a single flea.
Watch the top players using spider fac. THEY USE FLEA SCREENS. If a flea screen was bad, purple players would not use it.
Therefore, and to recap: BUILD A FLEA SCREEN.