Now that I read the passage again, it makes more sense.
Glorfindel is guiding the others along, because even
Aragorn is tired by this point.
Plus, as Glorfindel himself says, the
Nazgul could be anywhere along the Road:
'Where the other four may be, I do not know. I fear we may find the Ford is already held against us.'
They
may be at the Ford, but could be lurking at any point; hardly a situation in which to send
Frodo -- and
the Ring -- off alone.
The other question: why didn't Glorfindel mount Asfoloth and ride with him at the final moments, is separate. As Radaghast mentioned, the scene is important both thematically and structurally. Frodo has identified the most important element of his quest, during his conversation with
Gildor: 'Where shall I find courage? That is what I chiefly need.' We see him finding the answer as the story unfolds: it is
within himself. The hero may set out with friends and helpers, and encounter others along the way, but ultimately he must stand alone against his enemy, in defiance and rejection. He may be wounded, he may be killed, he may "fail", but it's a necessary part of the quest.
Tolkien certainly believed this, since he replicated and reinforced it at the end of Book I, and in the scene at Mt. Doom.
In addition, as far as plot goes, Glorfindel clearly recognized the need to obstruct the Nazgul following closely behind; I suppose some other reason could be devised, depending on one's inclination. I found it satisfactory.
Turning to
Tom, as it seems half the threads have, over the years, I'd concede that
the Old Man Willow and Barrow-downs sequences could have been written without his intervention, but how would you address what, to Tolkien at least, was an important element in his conception of this world, as he talks about in Letter 153?
Also T.B. exhibits another point in his attitude to the Ring, and its failure to affect him. You must concentrate on some part, probably relatively small, of the World (Universe), whether to tell a tale, however long, or to learn anything however fundamental -- and therefore much will from that 'point of view' be left out, distorted on the circumference, or seem a discordant oddity. The power of the Ring over all concerned, even the
Wizards or Emissaries, is not a delusion,
but it is not the whole picture, even of the then state and content of that part of the Universe.
(Emphasis mine)
Tolkien wanted his readers to get at least a glimpse into "the whole picture"; I think Tom performs that very well, even if some of us find him, or the picture itself, disturbing or unsatisfying.