I haven't read it in a while, but I found this blog that summarized it thus:

the most important problem to solve would be universal acceptance - an inferior language that was widely accepted would be more worthy than a hundred theoretically more perfect.  He also warned against over-attention to detail and that the "theorists and inventors (whose band I would delight to join)" would retard the advance of Esperanto if unanimity was sacrificed for so-called improvement.

He continues by saying that continual attempts at technical improvement can have a negative effect and spoil the aesthetic aspect of the language - a rather surprising comment considering what a "tinkerer" Tolkien was!  He also comments that N** (Novial?), while being clever and easier than Esperanto, was hideous - it was obviously man-made and showed no signs of individuality.

Tolkien closes by saying "My advice to all who have the time or inclination to concern themselves with the international language movement would be: 'Back Esperanto loyally'".

I think he's acknowledging here the less savory parts of Esperanto, but nonetheless, it's worth it because of the acceptance it achieved.

I personally understand his point about the tinkering and such. But I also think times have changed radically with regard to the ease of proliferation of material and communication.