It's all a matter of tradeoffs. A design philosophy like that makes it really easy to tailor TG composition to suit different situations, and specialized ships tend to be a bit more efficient.
OTOH, my own design philosophy is that every ship should be capable of operating independently-probably not as effectively as in a fleet, or as a specialized ship in a fleet, but still far better than the "not at all" of a missileboat with no active sensor or a PD escort with nothing to defend.
Basically, the former and latter give you different types of strategic and logistical flexibility, while the latter makes your TGs much less fragile tactically, because losing any given ship is going to marginally reduce the group's effectiveness across a set of factors rather than sharply dropping its effectiveness in one.
Take a simple example: a TG composed of eight 6k ton frigates. One's OOB is eight identical frigates, each with missile launchers, an active sensor, and a basic PD suite. The other's OOB is 5x missileboats with no AS or PD, 2x PD escort with only gauss turrets and an anti-missile AS, and 1x sensorboat with an oversized AS but no offensive or defensive weapons. The first group is going to have some combination of weaker PD envelope, lower tube count on the offensive armament, and weaker/shorter-ranged sensors relative to the second. OTOH, if the first group loses a ship, they are guaranteed to lose 12.5% of their PD and missile capacity and their sensors aren't affected at all. If the second loses a ship, it's a 12.5% chance that they lose 100% of their sensor capability, a 25% chance of 50% of their PD, and a 62.5% chance of losing 20% of their offensive armament.
tl;dr: specialized ships are more effective on paper, against sharply inferior foes, and prior to taking losses; generalized ships are less effective at any one thing but can operate independently and can keep fighting until the last ship is dead without serious impairment to those not yet destroyed.
Personally, I like generalist ships because there's nothing stopping you from supporting them with specialist ships. Make a bunch of multi-role cruisers and back them with PD escorts. Send a system-wide sensor ship to spot for your carrier group. &c.