For me the J is largely beyond such considerations. How many modern navy vessels still employ backup sails?
How many modern navy vessels travel in deep space?
As long as you're traveling out in space, you'll always be collecting whatever hydrogen/deuterium or other gases you desire to collect.
But most starships don't in Trek. They usually employ navigational deflectors to
push such things out of their way, especially at velocities approaching lightspeed. They have to
actively collect those gases.
The fact is that all StarShips are running on a limited fuel supply and need to replenish once you get back to base.
The fact is that some starships may lose fuel quicker than expected--either during combat or some other crisis, The Bussard collectors are for
emergency situations when your fuel supply is either depleted suddenly or when its questionable you have enough fuel to
get back to base. Sometimes it could just be a case of getting enough to top off the gas tank just to play it safe.
Collecting gas with the Bussard Collectors only helps alleviate a portion of the energy consumption that a entire journey would take.
Depends on how far the journey is, If your car is almost at E, but you can siphon off a little bit of gas from someone, it might be enough for you to reach the nearest gas station. It's no different for a starship in distress. Even a
small bit of fuel might be enough to reach the nearest starbase. It might mean the difference between saving your ship or having to abandon it.
Short of getting lucky and finding a dense pocket or going into the atmosphere of a local planet to collect massive amounts of gas and processing it later.
Hydrogen is one of the most commonly found gases in the Universe. At impulse power, the collectors can gather a decent amount of hydrogen just cruising through what would appear to be otherwise empty space. At warp speed, considerably greater amounts can be gathered. Finding a nebula or some other celestial body with a high concentration of hydrogen would just be the most
ideal situation, one to be taken if the opportunity presents itself. In most disaster cases, that won't be an option.
But either way, it's a viable option for extending a ships Hydrogen/Deuterium supplies that is always running 24/7/365 as the ship is traveling.
Which is why the
Enterprise-J still has collectors. Maybe one day future starships won't need them anymore and can generate their own fuel internally (whatever that fuel might be), but they may not be there yet in the 26th-Century...