1) what is the difference between cruise, sustained and maximum? Maximum means a short dash, but what about the difference between cruise a sustained?
2) Does it make sense to have slower modern ships? Is there a reason that the Nova is 1/3 the speed (at sustained) as a Galaxy? It makes logistics and coordinating the fleet much more challenging, it also means that the Nova takes 11 days to cross a sector, whereas much older ships can do it in as few as five days.
To my understanding, the biggest difference between Warp Factors is the amount of power required for each successive integer - it takes a an order of magnitude more power to establish the field for Warp 6 than it does for Warp 5, as shown by the yellow line on this graph, though sustaining a field once you've established it takes much less power.
I would surmise that Cruise represents a speed that the ship can maintain for long periods of time - months, really. It's the fastest the ship can go without massively depleting fuel and power reserves.
Sustained is the ship's "normal" maximum speed - the normal top speed of the ship in normal circumstances, roughly equivalent to the nautical "Full Speed Ahead". You'll use up power and fuel much faster, and it's less efficient, but it's well within the tolerances and limits of the engines, so it won't damage anything.
Maximum is the top speed that the ship can safely manage (any faster may damage the ship, either by burning out systems or the effect of inertial forces on the hull), without regard for fuel/power considerations, equivalent to the nautical "Flank Speed" (which is faster than "Full Speed Ahead", but only really used in emergencies because it burns through fuel and pushes the practical limits of the engines).
As noted, the biggest issue with hitting the higher Warp Factors is power - a Galaxy class will travel at Warp 5 or Warp 6 outside of an emergency, simply because that uses less power and causes less wear on the engines. A high sustained or maximum speed is only really valuable in a crisis, and not all ships need those higher top speeds. The Nova is a short-range scientific survey vessel; it doesn't need those top emergency speeds. Deep space exploration vessels (such as the Intrepid class), combat vessels (like the Defiant or Akira classes), and the like (the big multirole explorers like the Galaxy that do a bit of everything) are the ones that will need high top speeds, especially if they're used as test-beds for new technologies.
I see the logic there and it makes sense, same with the logic of the Google self driving cars having a max speed of 20 mph (or something silly like that) inside a city you're not going to get above that, so the fact that your Buggati Veyron can hit 253 mph is impressive, but not practical.
However (and do note that the the context that I'm looking at this through is "during war time, how fast are these ships moving around"), the differences in speed are a bit more than than. Putting Warp 7 at 55 mph (highway speed for a truck and trailer)
One consideration here is that Warp Factors are similar to the concept of a Mach Number: Mach 1 isn't a fixed velocity, but rather a dimensionless quality that is the speed of sound in whatever air conditions (temperature, pressure, etc) are present in that situation.