In a vacuum, Scarlet Spider is just "ok." The basic stat line is fine but not especially efficient: for 4+card you get 8 damage or 4 thwart, or the usual "...and a block" of any ally. I think the designers assume that Web of Life and Destiny will be somewhere at the table, refunding you a card on the cost, which brings the efficiency much more in line with a typical ally.

If you go all-in on using the interrupt ability, you can refund most of the cost (or all, with WoLaD), with the option to sacrifice the last card for a block. Spiritual Meditation is very good, and getting a block for just a single card is very good, but where Spiritual Meditation breaks even immediately, Kaine will probably take quite a number of turns to reach that point. For that reason, it may be best to take a mixed approach, where you use Kaine for attacking, thwarting, attempting to draw cards, and eventually block.

The best reason to play Scarlet Spider is for synergy. That's one more Web Warrior working with WoLaD, Warrior of the Great Web, and Spider-UK; one more expensive Basic card for The Power in All of Us; and, perhaps the best synergy, it's a way to make sure you don't waste HP whiffing on Spider-Tingle. If you're playing those other cards, Scarlet Spider is a strong consideration, otherwise, probably leave Kaine on the bench.