To me this allegory is simply saying that as the searcher is a 'critic', that searcher does not want to be out of the place where the light is currently being focussed, i.e. on modernism. The searcher may personally prefer what is to be found in the darkened area, but he also does not want to be
found in that darkened area.
I understand what Shippey is getting at here, though it is not
always the case that the light only shines on modernism; if it did only shine on 'modernism' then Shippey himself would not have got very far in his own academic career! However, it does have to be said that the British academic and literary establishment is in general quite hostile to studies of Tolkien and related literature; Leeds University is a notable exception in that it features courses not just on the literature which influenced Tolkien but also on his work in itself.
But, I would not like to shun the 'light' totally
just because it rejects Tolkien's work which I enjoy so much; this
could be implied in what Shippey says, if we interpret his words as sneering. I do not wish to exclude myself from a whole section of literature just because some (and these are a minority, though seemingly a vocal minority) of those who like it or are critics of it happen to sneer about Tolkien.
I think ultimately it's all about being defensive. The literary critics have a vested interest in keeping up the status of their preferred fiction as so many of them write and publish it, and it is still rare to get a bestseller in that genre; even Booker winners do not always sell well. Likewise, the defenders of 'popular fiction' such as Tolkien have a vested interest as they wish their particular favourite to be seen as 'serious' and worthy of intellectual consideration; I know I do.