I think they are worth it in the early stages of the game when you're in need of some quick cash.

As FH said you can make a pretty penny if they're in the right spot.

For most, if not all factions, the right spot appears to be Timbuktu.

I tend to lose interest in them later when I'm rolling in so much cash I don't know what to do with it all...

They are certainly not strategically necessary in any way.