This really.
At the very top clubs, you want a balance of all three, because you're able to buy the very best players in the world. But for lesser clubs, it helps to have some idea of what you need and what the weaknesses within your team are and how to balance them out.
So you may have a central defender who has no pace, but if you set up your team to help cover that, then you may be able to take advantage of his ability to break attacks up before they start and then distribute the ball brilliantly on the counter etc etc.
Personally, I like rounded players at all levels. FM heavily penalises physical stats by assigning them a much greater weight within the CA. Better to have those CA points elsewhere for many players unless you're willing to play to a specialised player's strengths.