It's just about the only way to wage an sublight interstellar war at all (excluding one-way invasion migrations, abandoning a prior planet due to disaster, depletion, quest for Zion, etc) and I just don't see the impulse for it with any kind of real frequency in the first place. The fear/hatred of the Other is all well and good but to reach the point of expending resources to do something about it almost always requires some type of competition in some way first, otherwise it mostly just leads to isolationism. And it's damn hard to effectively compete over things with round-trips running it at a minimum of 10 years.