Why the Venture exists is a very good question at this point. It should be justified in Lore or mechanics in some way.
It is. It exists as a fallback option for any faction without access to heavy industry, so they have at least one ship capable of being an anchor in thier fleets.
Venture is
supposed to be the poverty option for cruisers, in the same way Buffalo II is for destroyers, and Condor is for carriers. And the same way that all of them are fodder for the player to beat on in the early game.
As to why you'd want to build it over other ships, you probably wouldn't unless it was the only thing of it's size you had access to. Eg; running a heavy industry without any extra blueprints, something that happens only if you build a colony right at the start of the game, and do zero exploring.
The Venture is actually a civillian ship. You need to waste DP on Militarized Subsystems or else you get ganked by pirate patrols due to 7 burn (without Military systems) and high sensor profile.
Plus without it you get huge costs due to it being a civillian hull.
You can run from almost anything with burn 7 ships. I regularly build fleets around Colossi because they're the biggest thing capable of burn 7 without having to spend dock slots (which I want for other things), and have had no issues running when needed.
Burn 7 is not an issue for the Venture, unless you're obsessing about the highest possible speeds. In which case most cruisers will not be suitable for you either. This is quite an unfair standard to hold against one ship.
CivGrade doesn't increase costs in any way. Unless you're counting the increase from the military conversion, which again is unfair as it's not a requirement.