I would use frigates in my primary fleet if they had more peak performance and my fleet had a bigger cap (at least 40 ships to match the enemy). If some of my cruisers have peak performance problems in some endgame battles (due to how long fights are and how cowardly enemy can be at times), I would imagine my destroyers and frigates would probably be at critical if forced to fight in those battles.
Due to peak performance, cruisers are the smallest ships I use by endgame, except for Harbinger flagship. Even then, most of the cruisers I use are carriers, and most warships I use are capitals. If I need destroyer-grade ships, I bring and deploy one or two Falcons. I do bring Tempest and Shepherd, but that is mostly for enabling pursuit and auto-resolve (Tempest) and for campaign bonuses (Shepherd).
If peak performance was longer and fleet cap was bigger, I probably would use a more balanced fleet than big-ship only fleet.
Occasionally, I may bring a small Tempest-only fleet if I need to build-up rep fast during system bounties. Small group of Tempests will tear up smaller pirate fleets well enough.