Quick question about evolution

[Yoo]

Yeah I do get the basic reasoning, I just see a few practical issues. Back in evolutionary time when the spiders were just about to become venomous, they probably hunted by ambushing and jumping their prey. (not sure about this, but I think ambushing was there before venom or making a web was.) Now if you've ever seen a spider catch a prey there's not a lot of a fight going on between it and its prey. So that's why I have some doubts about the whole "weakening the prey is a real advantage" claim.

Any advantage is a "real advantage" if the subject is evolution.  That being able to weaken prey is an advantage isn't a claim...it's an observation of reality.  Sure, it may be overkill in some cases...but you don't want to risk losing a limb in an unexpected struggle with a particularly nasty snack..not like they
have an ER to go to, no such thing as a spider leg brace, eh?  Even if you're many times bigger and tougher than a spider... a komodo for example, you don;t want to -have- to fight tooth and nail to eat (or to keep from being eaten).  One bite...then wait.  One bite, then run.  Bonus points if your venom helps you to digest your dinner (as it does in the case of many spiders).  

At the population level, the less likely you are to die in any engagement..be it offensive or defensive...the more offspring you are likely to produce.

Mmmmm, I get what you're saying. But in spiders, the first of the venomous ones weren't yet adapted to having their own venom. So they would likely just hunt exactly like they used to. Which is jumping prey and killing them very quickly. In such a short fight, how could a weak venom ever come in handy? Same for self defence, a bird catches you, there's nothing your weak venom will do for you. 
I think I just found where I went wrong. I probably shouldn't think in terms of a gradual change in venom-strength. A (venom) protein has a very specific effect. The synthesis of a protein happens from your very dna, so a mutated spider can easily stumble across a protein which incidentally has a very strong effect on certain other animals. Like blocking the blood flow or something. There's probably not a gradual chain of ever increasing strength in venoms, because changing the protein sequence probably renders the venom impotent. There's probably a lot of non-relatedness (in the sense of not being anything similar in protein sequence) among venoms. 
So the first venomous spider might not have had a 'weak' venom, but already a very incidentally fatal one. So that solves the problem I had with your answer.
Thanks for helping me out!