Consensus on mentor usage and replacement.

I don't even train mentor. I remember always reading that it doesn't pay itself back in xp until a decent few skills in.
And since I don't play enough to get crews to that number of skills, I always considered it completely worthless.

I am always training it and swapping for other more useful skills/perks simply because it trains faster when low in skill counts. To me that's the only advantage and even then I am kinda doubtful.

I train it first, then swap it for 6th Sense just to get it that much faster. I'll train it again, and swap it for BIA. Then I'll train it a third time, and keep it a while while I train a few other skills/perks. Eventually I swap it again, and I will stop training it.

I agree with those who train it first. My understanding is that it trains faster so I will make it the first perk and then swap it for 6th Sense, then train it again, swapping it for Repairs, etc. It is a good skill to overtrain before moving your crew into a new tank

What you're doing with BiA is more appropriate for Mentor. BiA doesn't do anything till 100%, so swaping it straight out once trained it pointless.

Mentor trains itself, to requires less xp to get to 100% compaired to other skills.

Pretty much what I've started doing. 

I'm in between the 2nd and 3rd answer because I train Mentor first then swap it for 6th Sense, then I'll train it as my 5th crew skill after BIA, repair and either snap shot or track maintainence.

How do you use Mentor? I mean, it is an impossibility. Mentor is objectively useless in combat! How do you even use it?

I have a suggested use for mentor. If you already have it, drop it for something you can use. If you don't have it, find something in the skill section you can actually use. If there are only useful perks, drop an easy to retrain skill for the perk, and train the skill again. Steer clear of the uselessness of mentor.

In case you didn't notice, I'm the dude who voted for the first choice: "How do you use Mentor?"

25% not 20% i think but thats not the point.
yes for a while you have a 'dead slot' as you put it but its not about slots, its about xp needed to get your crew the skills you want them to have.
mentor will get them slots filled quicker and after time you will reach a point where due to mentor you will have 1 more skill than without for the same investment of xp. after that point its all gravy.

is it worth it? well thats what this thread is trying to answer.

but dont forget that dead slot with mentor in isnt really a dead slot, once you have most of the skills you need and there is just one left to get why not swap mentor for it? no more wasted slot and your target skills faster and for less xp than without using mentor.

theres more than 1 way to skin a cat, some better/faster than others but the cat still ends up dead.
edit:the comander only really matters on pc and this is about console. training dont work the same, a crew is 1 not many. also get the 25% elite bonus and stuff that pc dont get.