I find them very handy in entrenchment - especially as TEC - means I can build another SB - more trade - maybe a Frigate factory - had a game the other day - jumped through worm hole with constructor when the other guys was busy and got into the dead astroid nearby without him noticing - untill my 20 Kodiaks, 30 LRM and 15 carriers came out....... 

Yes,yes they are.

In what games ( entrenchement, diplomaacy, trinity and rebellion) is the Development Mandate availible?

I happen to have had a game not long ago where I was able to establish a super-trade lane going all the way around the system in a gigantic U-shape and the key was a dead asteroid (was at the lowest point of the U).... dropped a starbase there. was pretty awesome and was at least 9 ports long.