You are right, of course. Multi-role ships _are_ possible early game, they are just not very efficient (IMO)
My early game DDs are usually around 6000 tons in size. I have missile DDGs, beam DEs, beam DDs and AMM DEs. Now, combining those four types into a single cruiser at around 24000 tons would quite easily be possible. Problems with that approach are:
1) Forward bases are much harder to establish, as I need population and maint. facilities for 24.000 ton ships instead of 6.000 ton ships.
2) Assume I am in a war with a beam heavy enemy. Increasing the DE part of my fleet is not necessary, so I concentrate on building DDGs, which makes increasing my fleets firepower much easier than if I had to build large mulit-role ships.
The same goes for a missile-heavy enemy. I just churn out more DEs.
3) Shipyard can stay at 6.000 tons and can be cheaper/faster retooled for an upgraded class _and_ can build my ships faster (I am able to build 4 x 6.000 ton yards while you build 1 x 24.000 ton yard), so I have as much firepower as you do under construction, only my ships will be faster to build, so I can chrun out more firepower per year than you do (yes I know, a single 24.000 ton yard does not quite need as much time to build as 4 x 6.000 ton yards).
4) More targets for the enemy to shoot at. There _will_ be overkill if the enemy uses missiles, the more overkill, the longer my ships that are not yet engaged can shoot back.
5) Most importantly, if you have offensive beams and missiles on one ship, half of you offensive mass is wasted. Either you are in beam range, than your missiles are (more or less) useless or you are in missile range, then your beams are out of range. With a specialist ship approach, you can tailor your fleet to whatever is needed.
I certainly don't defy your logic or reasoning that specialist ships may actually be useful and might even be better at times. But when it comes to my cruisers being specialized that is mainly done through missiles and their hangar space. All ordinance can be specialized for so many things.
As I understand, bigger ships is also built much faster per tonnage so once you have those big yards you can if you have the infrastructure and resources build more tonnage in a shorter time.
When it comes to forward bases I mainly use a heavy FAC centric approach (and lots of sensor stations for defense) and they are very specialized indeed and quite easy to maintain anywhere. It is also very easy to hide their bases since you don't need a very large population or you use a small maintenance ships to maintain them anywhere.
All my cruisers have enough engineering sections to last them very well from launch to their next refit (when they get overhauled at the same time). So they can stay on station for years on end, even without a supply ship if need be. I don't feel that I need to sacrifice all that much space to gain that ability.
Sure, it might in the end be more inefficient... but I never felt it was thus far.
My main strategy is not brutal power but it is knowledge of my enemies forces. So I invest heavily in my scouting and ability to detect my enemies movement and then devise a plan to destroy them in force.
All my ship designs probably have much more defensive capability that most ships I have seen on the forum (except fighters and FAC). I rather have my ship survive engagement than not. It's more of a role-playing thing I suppose.
In almost all cases I'm the one detecting the enemy first or using my acting sensors on recon ships to make the enemy make a move in a direction that I desire. Drones is also very efficient in giving away enemy positions. All big cruisers have at least one large launch tube for drones and they usually have a large array of many different types of drones to choose from. My big cruiser designs are not just guns and missiles, they have many other uses and can easily be equipped for scouting in force missions, battle missions and all sort of interesting role-playing missions.
Each 36000 tone cruiser have at least one marine company with a drop ship for recon on force on the ground. Sure, that might seem like a waste of space, but has come in handy at least once and it is fun when it does. I plan to have several companies on my larger cruisers.
I also never lost a single cruiser so far, their defensive capabilities seem to be way more than my position can handle so far. I had one limping home in a pretty bad shape once, but it made it back to the repair yard. Most of the time i can just avoid confrontation if I deem the opposition to be too strong.