Whew... it'd be a terrible sight to see Sam try and hold his own against one of those. Hell hath no wrath like the fury of Samwise Gamgee at his master's injury, but stabbing spiders is a tad different than battling flame and shadow.

RE: Shelob cat of Sauron

Shelob was doing the job Sauron wanted. Why pay for another cat when the one you have is doing the job? Shelob was everything Sauron wanted, mean, nasty and vigilant. And he didn't have to feed her himself.