Supporting Saggittarii Late Roman

Have played a few games with these in my list and they have have little if any impact apart from giving more stability when a unit takes hits.

Are they worth it or am I using them wrong?

FD

They can shoot overhead of the chaps they're attached to without penalty Frosty .... which is normally a -2.

True but it seems a better option to equip the main unit with Plumbata and list the Saggittarii as separate S units.

Yes that's an option ...but as you've said they do also give the unit they're attached some added backbone at the same time.

Personally I far prefer them as the rear rank of a Large Unit.

They are protected shooters, firing measured from the front rank without penalty and as such as effective (but cheaper and more resilient) than ordinary T.

In combat they stiffen the unit just as much as a more expensive option.

there are always cheap alternate S options if you need a couple, but honestly unless you absolutely need to occupy a bit of terrain your battleline is best served by the archery threat of the rear rank saggitarrii