"Is there something more to [Shelob] that Tolkien may have mentioned?"
There's a lot more and it tends to meander into some rather esoteric territory, so bear with me...
The ahem inklings of an answer begins, quite literally, in what inspired Tolkien to write LotR: the enigma of éarendel from Cynewulf's Crist.
éalá éarendel, engla beorhtast,
ofer middangeard monnum sended
(hail earendel brightest of angels
above middle-earth sent unto men)
Reno E. Lauro states that "[a]rguably, Tolkien's life work can be summed up in one question, "who, or what, was éarendel?"
To understand the concept of 'evil' in Tolkien requires seeing beyond definitions of evil and to understand evil's place and function in his larger cosmological legendarium.
In this way two concepts are critical: Tolkien as a philologist as influenced by Owen Barfield's 'archaic semantic unity' (in his Poetic Diction) and Tolkien's interpretation of the medieval concept of the 'theory of light'.
Basically (and this is an oversimplification), the former stresses the importance and complexity inherent in the roots of "modern" words. The latter, in the place light has in the medieval world's concept and practice--in art and craft--of the 'good' (note: this involves both sacred and secular understandings).
Tolkien presents readers a world fractured as light is re-fractured. Here denizens present across a spectrum--each fractured as it were--from light to dark, but all as a whole awaiting unity.
The role or purpose of the characters is try to return the world to a semblance of unity which, of course, some see as light and some of darkness, yet some force striving for whole and, therefore, all characters playing a part.
Tolkien's story is, then, aimed toward a recovering of "an originary, undivided and mythic sensibility of light" (Reno, p. 54; see also Verlyn Flieger).
Tolkien believed that we (the characters) "can imaginatively craft the world into forms of beauty and harmony or bend, twist and consume it in the service of immediacy and power" (Reno, p. 64).
It is necessary to understand this to be able to adequately answer your question: "Is there something more to [Shelob] that Tolkien may have mentioned?"
In the end, although Tolkien stated that Shelob is 'evil', it is more important--and interesting--to understand her function as one of the fractured creatures on one side of the entire spectrum of light within Tolkien's world.
For Tolkien, creatures like Shelob demonstrate the part that evil necessarily inhabits within the whole of the spectrum: without having hope of redemption the world cannot return to being whole.
With this in mind, think of Shelob in light of Gollum's more major--but not dissimilar--part in LotR:
Gandalf: "...Do not be too eager to deal out death in judgment. Even the very wise cannot see all ends. My heart tells me that Gollum has some part to play in it, for good or evil, before this is over. The pity of Bilbo may rule the fate of many."
References for further reading:
- The Mirror Crack'd: Fear and Horror in JRR Tolkien's Major Works, Lynn Forest-Hill (Eds.)--especially 'Reno Lauro's 'Of Spiders and (the Medieval Aesthetics of) Light: Hope and Action in the Horrors of Shelob's Lair' and Rainier Nagel's 'Shelob and her Kin: The Evolution of Tolkien's Spiders'
- Tom Shippey's 'Orcs, Wraiths, Wights: Tolkien's Images of Evil' IN Roots and Branches: Selected Papers on Tolkien, Tom Shippey.
- Patrick Grant's 'Tolkien: Archetype and Word' emphasized text IN Understanding the Lord of the Rings: The Best of Tolkien Criticism Rose Zimbardo and Neil Isaacs (Eds.)