one advantage over the normal tiles is that is has a gold. That in itself is a bonus, but it also means that a golden age will yield a lot more gold since more tiles are effected.
for example (late game) you have a desert hill with mine (4
) and a farmed floodplain (4
). Change both to Kasbah and you end up with: Hill 3
1
1
and a floodplain with 1
3
1
. So a net bonus of 2
and two more tiles for a golden age yield increase.
So yes, i do feel they are worth it and it's just a matter how much food to production ratio you want, but you always have more gold.