I would not take the Monolith in this list simply because--short of grabbing an objective--it wouldn't really do anything. If you throw the twenty Warriors into the open, you'll phase against most opponents (after all, if they can knock them out at range there's nothing you can do, and a CC army has only to utilize a bit of movement to go around those Spyders and into it with your Warriors--transports would make running interception with your Scarabs difficult). They could sacrifice tons of men and resources in each combat because they know they'll win the game regardless.
In my opinion, you simply don't have enough Warriors to add them into your strategy, and given that, your second Lord is pointless. An extra 140 points to give them Res Orb will not change the problem: you don't have enough. It's a cool idea to go up the flanks and have the Lord and Spyders assisting phase out and all that, and it won't be as big an issue for some armies vs. others (Tau have trouble dropping Warrior units in my experience, for example, whereas IG could clean this list off the field in two or three turns without even an abundance of luck), but in general those Warriors are your codex-given weakness, your Achilles Heel if you will, and because you have treated them as such (investing so little in them), you need to protect them, not throw them into the fray. This is why I suggested the C'tan: it might pay to take one Lord to camp in the backfield with the Warriors in case of an onslaught of indirect fire, or to carry them forward if you have a Deepstriking enemy and need to be protected by the Spyders, but using them aggressively, especially against ranged adversaries, seems like a good way to phase out early.
As for Spyders, I have to concur Necroncell that I have not had too much luck with them, but I know some people who have, and I do see some holes in your logic. First of all: what do grenades matter? The things are initiative 2, even going at initiative order (I believe that's how frags work now?) they're still going last. As for probably dying before attacking a squad, the real asset of the Spyder is not the WS or the I (which are obviously their lowpoints), it's their S and T. You say they'll die before doing damage, and there are opponents for whom that's true (I don't take them much because my main adversary is Tyranids, and with their insane new book everything is either S 6 or small and wounds on 4's anyway, making Spyders and often C'tan a waste of time), but a Marine squad shouldn't be dishing out high strength until I1 attacks roll around (ie Powerfist) and even then, since you've thrown three Spyders into the mix, they should theoretically mess that squad up (about 5 dead Marines while the unit is forced to divide its attacks which wound on 6 in every direction and they might kill one Spyder via the Power Fist, follow up with 2 or 3 dead Marines the turn after).
Bear in mind, I'm not in disagreement with you, really: I don't field Tomb Spyders much, but I consider that a function of my opponents more than anything, who are generally fast, ridiculous in CC or so weak in combat that the Spyder isn't really warranted. But I have been surprised before. The same with Wraiths, I think a big part of seeing their potential is taking a lot of them. When it comes to the Wraiths, I think this is a mistake (after all, fluff-wise they aren't really supposed to be shock troops), but in general when you have so many that its guaranteed some will survive to dish out the hurt, units become more feasible. Our friend here has taken 9, which tells me not only that he's going to see what these guys can do, but also that he's sold on the idea of a Spyder list, so the advice of "Don't take Spyders" is sort of a waste here: it seems to be the theme we're dealing with, so unless Blindmage says he's not really sold on it and that was just a list to spark discussion, I figure we should factor that in.
That said, Gutstikk and Moc, the two biggest advocates for Spyder lists that I know, haven't showed up. Figure they'll have better advice than I (just speaking as someone who's never taken more than three).
