I would not go crazy for it, but if you have only got 8 packs left, you might as well as power through it. The Duel-A-Thon event should give you enough gems by the end. Sphere Kuriboh is still one of the few cards that could save you even against Anti-Magic Arrows and other anti-spell/traps. I would prefer not needing it in the first place tho...
Not especially low. On average, there'll be 1 UR left in the last 20 packs (200 packs, 10 UR's). So that's what, a 2/5 chance it'll be in the last 8, or about 40%.