[Kalimac]

This is definitely an interesting can of worms - or blades. I'd always been under the impression (for no real reason, I'll admit) that the Nazgul's Morgul blades were like their rings - they got one each for the duration and that was it, and since the blades work on the principle of a bee's stinger - one use only - they'd want to be careful about when they used them. The Nazgul are spending several months far away from Mordor, hunting for the Ring, so if they use up a Morgul-blade carelessly they don't really have time to go galloping back to Minas Ithil for refills, so to speak, so they'd have to be careful.

I can't see any reason why Sauron couldn't make more Morgul-blades, but the fact that, after Frodo is stabbed, we never see the Nazgul using one again, does suggest that perhaps they only got one each, or possibly only the Witch-King got one and the other eight just had to make do with ordinary weapons. (And it's hard to imagine the Witch-King not using a Morgul-blade on Eowyn if he had the chance, especially after his threat NOT to kill her, but rather to take her spirit along for some agelong torments - don't remember exactly how he phrased it).

If we accept that Morgul-blades were that rare, the question of why they didn't use them on more people would be clearer. The last thing you'd want as a Nazgul would be an enslaved (non-Ringbearer) hobbit-wraith, of all things; they undoubtedly would be almost useless servants and wouldn't be able to tell you anything that you didn't know already. The Dark Lord is nothing if not efficient.