Which still comes back to the question of are the skills actually part of the code of the engine?
If they are, and the balance of skills is generally retained, then I don't want a team of MCs who have a predisposition to Crossing, nor do I want defenders who don't like to gain tackling.

If that's the case, then I would want to find replacements for them.

If it's not based on the actual skills, and is based only on the overall value of the segment, then I need not worry about the cosmetic affect of the skills, and my fast trainers (who like crossing) will simply be better players because they improve that segment overall. I don't know the code, and sure it loads in a browser or on a phone quickly so is not going to be a Football Manager style game, but if the nuances can make a difference I'd like to try and learn them.

I'm not asking to create a Robben specifically as I can see that's not possible, but trying to work out if these players I have bought are in fact going to be worthwhile in the end (with additional crossing/aggression).

Look at it this way:
Option A) Points on different skills are immaterial, and only overall Defence, Attack & P+M skill levels matter. Fast Trainers = Fast Trainers and better long-term value
Option B) Different skills play a part in the match engine. Fast trainers are good, but
b1) If the skills additions are random your prized fast trainer could turn out to be a turkey in the long run, and you'll not get full resale value
b2) If the skill additions are kept in a close ratio (per 100 skills) then you will be able to buy with more information of his future position
b3) If the skill additions are pre-determined (but not in a ratio) you cannot know if you're buying a turkey or not.

so far, there's been no concrete answer to say if it's A or B.
And then if it's B, which way round.

My current hunch is that it's probably A, and that it is all "for show", and whilst I'll accept that, it's still a shame to have named them. My current reason for my hunch now is due to the answers given so far, the general ambiguity and the lack of dedicated goalkeeper skills.