the venture isn't a mining ship, really. the shepherd is a mining ship; it's primary purpose, is you know, shooting the rocks and extracting the goodies. the venture is a mining ship, but shooting the rocks is probably an afterthought. what it's truely for, being a cruiser is:
carrying the flag of the mining/trading detachment (commanders, officers, skilled personnel, VIPs)
protecting those people (strong armor, reasonable defensive capabilities, fighter bay capable of force projection)
discouraging piracy by defending it's subordinate ship(s)
carrying supplies (food, weapons, personnel, extra weapons, maintenance material, fuel, etc etc)
it's a cruiser after all. it wouldn't even probably be deployed without a fleet around it. too valuable and unwieldy.
but saying "it's just a civilian mining ship" is pretty silly. ventures probably form the backbone of many 'trade' fleets. even perform as mobile headquarters for entire corporations. a single cruiser is probably pretty rare, fluffwise. i wouldn't expect that you would ever deploy one without reasonable defenses and a supporting fleet (even if that fleet is just civilian stuff)
the game shortchanges all this by making ships far less rare than they probably are. cruisers die in the dozens in any major engagement, just because the game infinitely spawns cruisers and thus they are not of great importance.