like many above said: fun ("character") with a reason (depending on the task).
i'm very much a small ship cmdr, while having access to all ships ingame due to ranks and credits. i'm owning a dozen of ships, my to-go ship is a cobra mk iii, if not my python, i don't do a lot of combat, if i do, i take an eagle, dbs or courier. i love to fiddle around with outfitting and find a way to make a ship usefull for me (like the dbe...)
speed is not so important for me in terms of combat, but for several reasons: a) being able to choose your fights or disengage b) smuggling: running if being interdicted, or getting through the slot or to the pad fast c) missions: can you get docked fast and fast out of masslock.
there is a critical speed you need for survivability.
the cobra mk iii does this most efficient, but for various task (combat, longrange smuggling...) you might choose another ship of 10-15 mio, with different tradeoffs.
what i like about dbs, courier, dbe, t6 and cobra that they all have their strength and weakenesses. for exampel speed, shields, hitbox, number of internals...
till know i fail to see reasons to take a mk iv instead of the ships above. the additional internals, which are great (and maybe come usefull if mission cargo requirements go up) are very much countered by jumprange and speed. i also fail to see the point of an Asp scout in there.
i was very much looking forward to two new ships for my fleet, but both need more speed. not necessarily a lot, but somewhere in the vulture/Asp explorer range, with the cobra mk iv being a little faster for lore reasons, and the Asp scout havin more jumprange.