To answer your question:
It would afford the player more granular control over the composition of their fleet, while maintaining a reasonable limitation they must work within. It would allow the player to compensate for the current shortcomings of lighter ships (which are generally reasonable) with numbers, making frigates more viable throughout all stages of progression. In other words, it sidesteps the fact that a fleet comprised of 30 frigates will almost always be outlasted by, or will otherwise lose against, a fleet comprised of 30 capitals. It removes that conundrum from the decision-making process in regards to composing a fleet; Instead, you'd be able to have any number and mixture of ships in your fleet, up to a limit of overall strength - instead of being forced to compromise that overall strength for a more diverse force, or compromising the diversity of that force in the pursuit of overall strength.
It'd also be consistent with the behaviour of deployment points - The stronger the ships you deploy, the less you can deploy at once; The weaker the ships in your fleet, the more you can take with you.
I maintain that a change to fleet cap wouldn't factor into issues with the number of simultaneously deployed ships, as that is controlled by a seperate value (Deployment Points). For the record I think the current deployment limits are as good as they're going to get in light of the AI's behaviour, and general performance; A change to fleet cap would simply mean changing how many of which types of ships you can have in reserve for a given battle.
In other words, the relatively short PPT/CR/Survivability of Frigates doesn't mean so much in the face of that of Capitals, if over the course of a battle you're able to deploy, retreat and reinforce as many Frigates it takes to equal the overall strength of that Capital. It also means the fleet you're fighting can't outgun/outlast you simply by having heavier ships on average - because to have heavier ships, they'd necessarily have less ships.
Currently, those reserves are best comprised of the heaviest ships possible because you can only have up to 30 total, railroading the player into a general fleet composition, at penalty of often being outgunned by NPC fleets with similar numbers of ships.