I only ever rocked with 2 science ships ever since I've played Stellaris (one for archeology and one for surveying). The cap still gripes me because now certain leader types, such as Generals and Governors, are basically a waste of a slot now. You can't have multiple fleets... well you still can... but you'll have to stomach the XP penalty that will come with it.
Generals, I'll agree on, but not so much governors. A level 1 governor gives a flat +2% bonus to all resources on all planets in the sector and -2% to empire size cap. Start stacking on traits, and they're a pretty steady bonus to whichever sector they rule. Industrialists are particularly nice for their sectors: another +5% bonus to resources at level 4 from the veteran trait, up to +15% food output, +20% slave output, +35% to industrial outputs (alloy, CG, or refineries), +15% to researcher output, or smaller bonuses across the board (+5% ruler, +10% specialist, +15% workers) depending on which traits they roll. Pioneers can pick up +45% to various raw resources with investment, and Visionaries provide not only bonuses to unity, but they have the chance to provide one of the few flat bonuses to Influence. A high-level governor with decent traits is not only superior to a planetary designation, but stacks with that designation and all other bonuses you might have picked up.
I'd consider them more useful than Admirals, who are only good in times of war, as governor bonuses can feed into whatever goal you desire. Apart from the bare minimum to fill in whatever council slots absolutely mandate military leaders, admirals tend to be a distant third in priority for my play style. If I can't afford them, I'll simply run fleets with no leaders and convert those bonus alloys into even more ships.
The biggest issue is the random nature of the level-up traits. It can be moderately bothersome to have a sector of foundry worlds humming along only for the sector governor to end up being given the choice between something like food production, research, or trade.