When to use thimbles instead of blocks?

[Brion Toss]

Hello,
You're talking lizards and dumbsheaves here, which is to say that the concept of a no-moving-parts purchase goes back a long ways, and that it might be helpful to look at antique applications to inform contemporary ones. While it is true that high-load, short-travel doesn't always need friction reduction (deadeye lanyards, for instance), there are instances where relatively low loads, over somewhat longer distances are also done sheaveless ( sending topmasts up, for instance). I think of it in terms of how much bother the friction is; it seems likely that sheaves are the product of frustration, an invention that, complex and difficult to make as it is, is worth it for the labor it saves and/or the efficiency it gains.
And this is part of a larger progression. No purchase at all is simpler than a purchase through a lizard, and the lizard is simpler than a sheaved block, and the block is simpler than a multipart purchase, and the purchase is simpler than a hydraulic setup, which represents a complete departure from the previous tactics. And alongside this progression are parallel progressions in material weights, densities, elasticities, durability, etc., and all of those are entertained as possibilities to the extent that they seem worth the effort/expense.
Therefore what we are talking about is good ol' design, whereby we consider what loads we have to deal with, what resources we have available, and then go about selecting as we think proper. So the more you know about the advantages and limitations of lines through thimbles, the more likely you are to make the best use of them.
On a practical note, one limitation of the setup in your photograph is that, given sufficient run, load, and time, the line will wear a groove in the thimble, increasing the friction and threatening to saw right through. And the process can be invisible, so lift the line often to check for wear.
Fair leads,
Brion Toss