I was talking more in regards to personal fleet diversification rather than combat fleet composition where server instance limits would apply. Basically having flexibility on what you want to do at any given time and having ships to fall back on for when you're waiting on replacements. When you give up multiple smaller ships to contribute to a large ship, you have less role flexibility, and are subject to the burdens of the larger ship.
I mean, it sounds awesome to own one of the massive ships, but when you get right down to it, they're more of a liability than anything. Longer replacement periods, large crew requirements, high profile target for pirates, etc.