Yeah, Algeria is rich and can fund an army by itself (eg: useful for when a crusade is successful and the troops then start to cost you money!!).

If you don't take Algeria, the Sicilians probably will, unless they've become involuntarily isolated due to loss of ships, or wandering ship syndrome. You don't really want low-zeal Christians between you and Egypt/Outremer. A bit inconvenient.

Secondly, the Almo's Urban Militia are extremely annoying and heavily armoured and you should relish the opportunity to finish them off once and for all in a nice hot desert environment where they will bake like spuds.