[_Kila_]

I think it's also worth mentioning that the mentality these fortress gals provide is very positive. People are far more willing to DC in a gal full of their alliance members, and you don't get silly little conflicts/situations where you can't defend each other. It's much more of a unit than a regular gal, you go through the same shit, if one of you has incomings it's likely that the rest of you also have incomings.
It also makes DCing easier IMO. In round 30 we had one or two people DCing for each gal and then JBG/Golan picking up the pieces with planets that weren't in fortress gals. It made the nights where all our gals were hit by that massive block so much easier to manage.

As people have explained, it's a strategy that also has drawbacks and making fortress gals is a risk as they can fail quite hard if the alliance/players in the gal can't cope with it, as such I don't think it's a tactic that should be nerfed.