PH, you think locomotion is evolutionarily more important than foraging? I would think that the use of silk to create webs that can catch prey would be a more important evolutionary event; the use of silk as a means of locomotion being a next step to better situate the means of ambush, not the other way around. This can actually be supported by the fact that some species of spider create ground traps with silk rather than create webs that are arial.
What's more "important" is not necessarily the function that evolves first. What's "easiest" is often the first thing to evolve (i.e. involves the least amount of modification of existing structures or biochemistry).
Evolution usually proceeds by small stepwise modifications, and the first few small steps towards the development of modern silk and silk usage may not have involved the catching or holding of prey, because that would involve stronger silk and more complicated usage of it than one would expect during the earliest stages of silk production.
However, it wouldn't take much modification of existing insect physiology for an early insect to have had a viscous substance (a proto-silk, as it were) seep out of its feet in order to improve traction as it walked (rather in the way that snails and slugs currently leave slime trails). If that was the first step (no pun intended), from there evolution would have driven the stepwise improvement of the sticky substance itself, its applications, the behaviors which drove how it was used, etc., until much later it was suitable for creating "suspension bridges", traps, wrappers, etc.
That part sounds a bit hazy on the details, Ich. Kind of hand-waving.
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Speaking of hand-waving...
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Are there any species of spiders or closely related critters who "spin a web" and then lasso or similarly throw it over their prey? Might be a nice transitional between leg-silk and posterior silk.
Cheers!