You are right, which is why I think being able to buy an extra trainer seat will sell more units. Otherwise it goes unpayed for. As you state, who would want such a thing? Well it has one job, to train people. No noobie is going to pay $60 on a game they are unsure about.
There are three outcomes,
If they like it then they must buy a another $60 combat module.
They don't like it then they have wasted $60.
They skip the trainer and just jump to a $60 combat modules.
Now if you could pay $80 or $100, or $120 for a trainer plus learner seat then you can convince dozens to come try DCS.
You can host a trainer server and teach people. This module to a "Flight Instructor" now has purpose, it has a role even if non combat.
You may not buy a trainer because you want to fly CAP, CAS, or Logistics but those who would, would do so because "Flight Instructor" is the role they wish to play but if they can't convince new pilots to shell out $60 then they aren't able to do so and module loses it's usefulness and must be shoe horned into roles it's not designed for like CAS and CAP.
This is also an option. To be able to buy a trainee slot for $7-$10. Maybe a mixture of the two.