Because I haven't written enough completely unsupported articles about my general football philosophies lately!
When a team is defending, it makes sense to get as many men behind the ball as possible. The more men at the back, the easier it is to clog up passing lanes, to make challenges, and to deny space for attacking runs. Even the best teams find it difficult through a tightly-packed defence, as we saw whenever defensively-minded teams have faced off against Barcelona at the Nou Camp. Denial of space is an extremely effective technique against any sort of quick-passing offence, and it's the typical modus operandi for any team facing off against significantly better opposition.
So is pulling everyone behind the ball the optimal means of play if you're concerned about your defence? It's pretty clearly not if you want to score, ever. When the ball is inevitably won by the defence, they would faced with the unfortunate scenario of having to play the ball slowly out of their own half - with nobody up front there's no quick outlet pass that quickly changes the initiative and puts the team on the attack. If you're playing deep, having possession does not necessarily mean you aren't playing defence - just look at how difficult to deal with a throw-in next to your corner flag is.How about for pure defensive purposes? I'm not even sold that it works too well, to be honest. While parking the bus in front of goal undoubtedly lowers the chance of any one chance being successful, there's another worry - no matter how you slice it the frequency of attacks increases. Why? The secondary ball.
When defenders win the ball anywhere near their box and they're under pressure, they're not terribly inclined to control and make a clever pass to extricate themselves from their predicament. Indeed, they're not usually equipped to do so - most of the time they only have fractions of a second to clear their lines. What this means is that clearances are hit without concern for where they land, so long as they go in the vague direction of away from goal.
Have you ever watched a team lay siege to the goal? It seems like the other side simply can't hold possession - even when they get the ball away, it goes right back to the other side, and they launch another attack. To me, this is a symptom of over-defending, committing so many resources to stop an individual attack that none are spared to stop the next from even forming. Most modern teams station a deep lying midfielder to mop up long balls launched by the other side into their half, but too often that same principle isn't followed when a side is under sustained pressure. Be first to those clearances, and there is a chance to end the attacks, rather than just endure them.
Of course, you can't do that without committing players to sit in the mop-up areas. But with the opposing centre halves typically staying back on defence, a defensive team should outnumber the attackers by at least two players - less, if your centre forward is lazy. But there's really only a need for doubling up in defence at the focal point of the attack, and realistically you're never going to see all eight potential attackers pushing high upfield at a time. So if you're facing six attackers, why on earth would you need eleven defenders?
My answer is that you probably don't. I don't know for sure, of course, but it's clear that the amount of goals you should be expecting to concede goes like this:
Gs = A * B (Eq. 1)
where Gs is the number of goals scored, A is the average chance of an attack succeeding, and B is the frequency of attack.
It seems to me that many, many teams are committing all of their resources to minimise the first term while ignoring the last. Is that an optimal balance? Just from observation, I doubt it. Attacking is already so difficult that the difference between an eight and nine man defence can't possibly shift things around that much, but having two players further up the field rather than just one may well have a large effect on B.
You might make the argument that defensive tactics have been optimised by years of playing soccer, but in my experience the status quo has a lot more power than we'd all like to think. Yes, it's true that using that last player to minimise A might be more efficient than B, but my inclination (very slightly supported by maths) is that this is not so, and when I haven't seen evidence one way or the other I'm going with my gut.
Either way, it's obvious that Eq. 1 above is what defences need to be working on minimising, but what's not at all clear is whether they're actually doing so. My suspicion is that 'parking the bus' is often wasteful, and using just enough players to defend reasonably well gives you more bodies free to contest possession immediately after a clearance. Sacrificing defensive solidarity for 'just enough' defending may well help a team perform better overall.
Of course, there's nothing conclusive in the little thought experiment about, apart from the fact that Eq. 1 is clearly true by definition. I just think it's one of those things that's interesting to consider.