Just to add, I don't see anything wrong with the guides even during gameplay, personally. You're not getting assistance, you might be able to put the line on the player you want to pass to but an opposition player might still intercept, or the pass target might be running forward and not have chance to abort his run and come back.
I absolutely love the guides - I have them on even when playing assisted/semi just so I can see if I'm pushing the stick in the direction that my brain is intending (I'm shockingly far off sometimes)... Great addition to the game IMO.
Yeah, I'm very glad they are in too. I think it was PES2013 that was the first to actually a visual guide, but you had to manually activate it with L2. I used to tape down the button so that guide would always be showing lol.
Joonloop: I know its only visual, but it gives you a massive, huge advantage against people who have turned it off. Because you know where you are aiming

This shoud not be possible in Seasons. I have no problem with it in friendlies though.
If you know it's only visual then you should know that it has no bearing on controls being assisted or manual.
As for advantages, I agree it gives some advantage. But personally I think the Trainer makes the game a better 1:1 representation of real football compared to it being off. Because real players have more control of their body and accuracy then an analogue stick with no guide. With trainer turned on you still have to aim properly and be quick about it if you want to get a quick pass off, but it also provides you with the ability to better fine adjust some passes and shots, which I think is more realistic. Even the camera angles plays a big role in this - play full manual on broadcast camera and see how unrealistically difficult it is to play a match of football, compared to a real player who only needs to orient themself only around themself (obviously).
There are also different types of Trainers too. FIFA18 allows you see a long "guide line", for example. I still think FIFA16 has the best Trainer though, because the "guide line" was very short, but it also provided a circle around your player with 4 little gaps to allow you properly oriented north, east, south, and west, like so:
If you disagree that the Trainer make for a more realistic experience then thats fine and we could argue the merits of each position.
But if you inherently disagree with the concept of on-screen guides, then I would counter with the argument that things like radar, stamina bar, and player indicator are all already in the game and make the game better, because there is certain information that needs to be transmitted to the player and it can't always necessarily be achieved through pure "gameplay" - sometimes it just needs to be communicated through overlayed UI.
In first person shooters we use on-screen crosshairs to orient ourselves because in the game we don't have access to all the complex measurements that our own body makes in real life. Do cross hairs make 1st and 3rd person shooters more or less realistic? Do they harm or improve the gaming experience?