HOFi Giac's lack of rPois renders Spider:1 more dangerous than Shoals:1 and likely Shoals:2. Which is a significant swing, considering that Shoals is generally the most difficult Lairiable branch, and Spider one of the easiest, since it's mostly countered by a blowgun.
Worshiping Ely mostly mitigates the rPoisless Spider threat, since Purify neutralizes not only poison but also Tarantella confusion. Ely also counters Snake via Heal Other, which counters the fast snake harriers who fix for the slow nagas. Thus a major reason Ely is a common part of early essential builds is that she counters Poison.
The shibboleth that rPois isn't important for Spider is a disingenuous pseudo-intellectual pose that attempts to inflate the adherent's status by displaying his knowledge of an obscure detail of poison damage mechanics, which is supposed to imply his superior knowledge of the game in general. Like many popular factoids, it is anti-knowledge. Whether or not one has rPois is absolutely an important factor to keep in mind when planning for and playing Spider. It makes a major difference in the degree of aggressiveness that is appropriate.
In fact, it can be argued that rPois is more important in Spider than Snake, since Ely is better at countering Snake than Spider. Ely is stronger vs slow than fast monsters; spider swarms can overwhelm her.
However, the whole debate is moot. One will never need to decide whether rPois is more important for Spider or Snake in order to play DCSS. The relevant question is whether one's Poison branch is harder than one's Water branch, and this can be tested by doing a floor. In this light, believing that poison isn't "important" for Spider is terrible advice that will lead to underestimation of the difficulty of no-rPois Spider relative to Water.