Hmmm, well, I get the feeling that along with the inevitable casualties that will be caused by randomly jumping through the Void, there might be issues along the lines of not being very accurate in where they come out. So while it's POSSIBLE to travel without a void key, the directions you can give are very vague, and you can come out of the void just about anywhere. So if you make a small jump, super quick, and try to limit it within a single system, your just as liable to come out inside the sun as you are to be above the planet you want. I feel that it would work better the more data you have of the area your jumping inside, like the relation to the void for most of the system, but it's still very finicky.
The idea being, if you're in a totally new system and you try to Void Jump somewhere else in the system with no data, your exit point will be almost completely random. While if you have the ENTIRETY of somewhere like the Origin system mapped with this system, you could use it to travel literally anywhere within it at any time, but you'd have to have ALL of the data and keep it constantly updated at all times due to the constant shifts in relation to the void.
So it's GREAT for jumping at random to find a new system if you don't mind the horrendous casualties that you'll incur (which the Grineer don't), but practically useless for in system travel, and it's honestly easier to rely on the Rails once you have them built than it is to spend the resources to keep it up to date.
To make a long story short, if you plan on doing this, it shouldn't be a very good method of travel.