[js1138]

Also, why would a race use the extremely valuable resources of their solar system to create one-way journeys that would give zero return for their effort?

Possibly because intelligent probes would have their own reasons.

[Robert A Cook PE]

Your question beings up a critical point:

Travel, of course, must be possible to "start" the idea of a transstellar civilization.


BUT ...

Two more things are essential:

First, you must be able to travel BACK to the otigin. (Requires navigation and at least a two-way amount of supplies and resources/fuel. (Worse, you don't know whwre you're goinf when you've left: How many stars are you going to have to aim for, travel to, de-accelerate into, explore, then accelarate away to the next vefore you find ONE important enough to want to return home from?

And, once you've left your origin, you must be able to navigate back to where your origin NOW is when you've gotten back there. (Rotation of the galaxy and all that minor stuff.)

Coomunication is as important as travel: If ou can't communicate faster than speed of light, nothing you do away from the home system will ever be economically viable.