Spain allow their opponents to focus entirely on maintaining their position, and this makes it difficult for them to break down even fairly unimpressive opposition.
I would word this slightly differently personally. Spain allow their opponents to focus entirely on RECOVERING their position, plus the rest.
They will pull you all out of shape if you let them, but because their progress from front to back is usually so measured then it allows teams to get out of shape but often recover.
It has the effect of feeling like a training exercise, Spanish attack vs Opposition defence from 40 yards out. Defence tries to stop attack getting through and when it succeeds, they just throw them the ball and the exercise begins again. The 75 yards of the pitch from Casillas forwards is almost irrelevant to the game.
Whether you like it aesthetically or not is one issue, quite aside from that is just how at odds it is from the rest of the modern game. Because organisation and shape at the lower end gets better and the physical gap between fittest and least fit narrows too, the focus is on counter attacking and trying to gain a numerical advantage over opponents. Spain and Barcelona utterly go against the grain of accepted modern tactical wisdom. Their desire to plough their own furrow is at least commendable.
If you pick a centre forward, you run the risk of losing possession, but it destabilizes the opposition, and makes it difficult for them to maintain their position. Basically a centre forward moves the centre halves around.
If you pick a centre forward in the traditional sense then you are losing control and unpredictability. You know where he has to play, I mean he might drift out wide on occasions but as you say, by default he is there to get among the central defenders.
This means that your opponents can control where he plays. If their defence pushes up the pitch, then the striker is pushed deep. If they sit at the edge of their own box, the striker has to be high up the pitch.
This has the net effect of stretching your team too. The centre forward moves as much in line with the opposing defenders than he does based around the movement of his own teammates. Now one of the core aims of this Barcelona and Spain side is to have as short a distance between front and back as possible.
This is why they favour ball playing defenders who could play midfield for most sides, also why they favour forwards who can drop deeper and play as auxiliary midfielders. They create a narrow strip of pitch with all outfield players crammed into it, they know that no other team can knock the ball about congested areas like they do and this reduced area of play cuts down the work they have to do when they want to win the ball back through pressing.
That's the theory behind it all and personally I have no issue with it in principle. In practice I have my doubts at the moment though. It works with Messi so well because he can do anything, anywhere.
He can drop deep and can play pass and go's, but he has the pace to run on to them. Similarly, if he is tracked he has the skill to beat players at will and overman the opposition.
Fabregas is a fine player but he cannot really do either of these things. There is no-one in this Spain squad who can and I think this is the only real issue they have to address. Calls for Llorente would not help this, he is perhaps the answer to a different and much less significant problem.
The best metaphor I have seen for describing Spain's play is death by a thousand cuts. Every pass, every exchange, every touch inflicts a small amount of damage or physical and mental fatigue on their opponents. After so many minutes and so many passes of this, the cumulative effect adds up and their opponents weaken. It's almost like bullfighting in concept.