yea, the way I see it, if you are taking spyders in your list and are taking scarabs, plan out how many scarabs you want, then take 1 less scarab for each spyder as you'll be fielding. essentially a big discount on your spyders.
The reason I personally think spyders are so awesome is that they are only 10 more points than a lychguard or pretorian. with +1T, +1S, +2 wounds, MC status and the ability to pop out scarabs they are a steal. sure their WS and BS are a little lower but I don't plan to take their particle beamer as it is so expensive, and they will still hit anything less than 7WS on a 4+.
10 lychguard is the same cost as 8 spyders (the spyders are actually cheaper than them if you factor in at least 1 free scarab), the only reason I don't buy tons of these things is that they are in probably the most competative army slot, heavy, competing with the monolith, annihalation barge, doomsday ark and death scythe, all of which I consider excellent vehicles.
I think I'll probably be getting 3 more spyders to add to my current 3 at some point, they will be very good in low point games, letting my focus on infantry and having them provide my close combat power.