I guess it's always going to be a compromise and it's up to everybody to decide, for themselves, where they want to make that compromise.
To digress slightly, I always used to run a pair of C7 SCBs on my 'vettes and they worked but they were always the eventual reason why I'd have to bug out of a fight.
When I got rid of them, it meant I could remain in combat for as long as I want.
Thing is, "as long as I want" is a finite length of time.
That being the case, there's not much point in building a ship that can remain on station for longer than I'll actually enjoy being there.
A pair of 4A beams can explode 95% of the ships you'll ever see at a RES or CNB.
Once every 15 minutes, or whatever, an Annie or Corvette will show up and killing that will be easier with the assistance of MCs or FCs.
In that case, a 'vette with 4A beams and secondary projectile weapons can easily remain on station until I've got bored.
Things are a little different in CZ's, these days, now the ships there are tougher.
As a rule, I tend to find that I can win between 1 and 2 CZs before I need to synth reloads for my MCs.
Again, though "time to boredom" is also a factor.
If I was running all-lasers, I wouldn't be winning CZs as quickly so I might win 2 or 3 before getting bored and going home.
With MCs I can kill ships faster, win the CZ faster and, thus, I'll win 4 or 5 before I get bored and go home.
That means maybe 1 or 2 reloads so that's not a huge amount of mat's and it's acceptable to me.
At the other extreme, if I was running an all-MC loadout on my 'vette (and I have tried it) it'd mince opponents quickly but it irritates me to be wasting ammo' just swatting things like Eagles, Cobras and Asps.
For Assassination missions, that'd be a terrific loadout but for farming CNBs and CZs it's too wasteful of mat's for me.
Again, it's about considering all the factors and then choosing the appropriate compromise.