What ain't there and ought to be.

[Lalwendë]

Lalwende - You may be right on the idea of the "comparative safety" of the trench at least when considering the alternatives that JRRT would have had! In reality, however, they were muddy and unsanitary and unpleasant places!

The trenches were undoubtedly so! Tolkien was after all sent home with Trench Fever, which is contracted by suffering from lice, so we can tell he did not find the trenches too pleasant. Now, right at the start of The Hobbit, Tolkien actaully qualifies exactly what sort of hole a hobbit might live in:

In a hole in the ground there lived a hobbit. Not a nasty, dirty, wet hole, filled with the ends of worms and an oozy smell, nor yet a dry, bare, sandy hole with nothing in it to sit down on or to eat: it was a hobbit-hole, and that means comfort.

The words in bold seem to convey a little of the trenches - and maybe he made use of the 'comparative safety' element when creating smials, but also needed to make sure we knew, as readers, that these holes were something quite different to the filthy conditions of the trenches.