Maybe, out of habit, he doesn't want a powerful servant like a Nazgul to be tempted by the Ring.  Maybe he thinks a Hobbit-wraith would be a safer delivery boy.

1 – Of course, the $10 Million question of this chapter is: why didn’t the Riders take the Ring, take Frodo, or take both?!  It was right there!  A few theories:

- Strider’s opposition was too much for them:  granted, Strider is the heir of Elendil, but he is still only one against five.  About the same number of Nazgul wiped out an entire band of Rangers at Tharbad.  (You mean Sarn Ford.  I have to discount this one; the Nazgul are wimpy enough here already).

- The Riders intended to take Frodo, but wanted to give him the wound first to lower, and eventually destroy any resistance.  (Yah, I guess that's my theory above.  I am sure that Sauron wanted to especially torture anyone who dared to hold his Ring, and would want Frodo to be forced to personally hand it over; sheer meaness saves the day).

- The Riders only intended to wound Frodo, then leave him in the wilderness, watching and waiting until he became a wraith.  (If I were writing the book I'd hint that the Nazgul were so dominated by the Ring that they were very limited in their ability to act against its bearer.  It happens I didn't write it, so this is as good an explanation as any).

(personally, I lean toward a combination of #s 1 & 2)

2 – Frodo feels a terrible temptation to put on The Ring, which he finally succumbs to.  Were the Riders sending him that message, was it The Ring, or was it a combination of both.  We know the good guys couldn’t see Frodo after he put on The Ring.  What advantage did it give the bad guys when he put it on?

(We are told that the Wraiths might have seized Frodo.  No doubt they could see him clearly.  And it makes perfectly clear who's got the Ring).