They are employed in Tool rushing, yes, and I question their value.
IMO, these are the things in which they truly excel:
1. Anti-Bowmen unit
Bowmen, not long range missiles....
2. Great wall destroyers.
And silent and inexpensive at that (an ST costs a lot more...)
3. A great Tower killer.
Except ballista towers, of course ...
4. Not bad at tearing down buildings
Only if you have lots of them and no easy kills in sight...
IMO, the anti-bowmen is their main role, thus fitting the bill when facing Assyrian or Hittite bowmen attacks.