I've never liked this explanation--if it's just moving space at less than the speed of light, there isn't much purpose to those giant torchship rockets on the saucer section. Maybe Probert's warbirds do that
, but if there's no qualitative difference warp and impulse, 1)why call them separate things?; 2)why carry giant open fusion reactors, He-3 (or possibly H-2*), and propellant with you?And of course, without Alcubierrian magic, they aren't hitting anything like .25c with the propellant load they're capable of carrying in the volume that isn't occupied by people, air, holodecks, and such.
On the other hand, that does make impulse lame and only useful in very limited situations--I'd assume the major ones to be combat and moving out of the ecliptic where the majority of dust, intersystem shipping, and civil transporter beams are. But you don't need tons of speed to move out of the ecliptic in a reasonable timeframe, and even then it's hard to imagine why warp wouldn't be possible going solar north or south, where one would be unlikely to smash into stuff.
*What's the deal with the deuterium fetish Trek has? Especially antideuterium? Why would you make antideuterium? Why would you use it? Do they like electrically neutral antimatter fragments with ten minute decay times launched at near-c through their ships? Maybe I misunderstand annihilation reactions. : /
