I think this feature is intended to give some benefit to raiding and harassing attacks that can't take over a planet, but can maybe take out some structures. It forces players to either be very sure they can protect all the structures or to build some spares unless they can accept research and production delays.

The requirement for military labs to keep building cruisers can be rationalised as the labs being involved in arms production - the game reason of course is that to keep the load on the player reasonable for a real-time game the number of building types has been kept small.

On the whole I think the feature is useful to the overall balance of the game. However, it could be better explained to players either with the tutorial or the tooltips.

There's also an issue with the interaction between fleet logistics research and the other two research trees. There's not quite enough information on the fleet log screen to warn the player if they queue up research that will eventually deadlock their researchers and that's exacerbated when labs are destroyed mid-research. Improving the detail of the "lab destroyed/decommissioned" message would probably help there, as would adding information about available total labs to the fleet log research interface.

PS. I have to add that I was quite impressed with the unobtrusive yet effective way in which the other two research screens show how many labs are available.

Astatine had it right. This was a deliberate design decision to make it so that you couldn't research what you needed and scuttle the labs to build other stuff, in addition to making raiding a valid and effective strategy

On that note, it seems odd, that having 10 military labs doesn't let me research some lame T1 tech faster, or even 10 T1 techs at once... neh.