Military service is highly advantageous.
If you are playing as an empire with more than population (that includes robots, droids, synths) then it becomes valued even more so with the Evolution path.
Being able to force certain species to be your army and others not allows you to get the most out of your empire.
Here's and example:
You have 3 races.
Main Species: Thrifty (15% energy),
Fast breeders (10% growth)
2nd species:
Very Strong (10% minerals, 40% army damage),
Slow Learners (-25% experience gain),
Resilient (100% garrison strength, +50% garrison strength)
3rd species:
Intelligent (10% science output),
Charismatic (5% happiness to other species)
Weak (-5% minerals, -20% army damage),
Fleeting (-15 years)
Now as you can see. You really want your 2nd species to be your "fodder" units. They will be perfect as your army units for both defending and attacking. They are also not so great leaders because they suffer from a negative exp gain. Making their Military service great for just armies and not full service.
I don't think you have fully explored everything you can do in this game and just how helpful the numerous options we now have at out disposal for making an empire.