Arcadian,
I don't use the term 'footy' or 'rugger' unless I am attempting to mimic those you describe, but it doesn't really irritate me the way 'soccer' does!
On your point about which team people support, while I think your generalisation fits quite a few, and particularly people from outside the UK, for those in the UK it will be down to a lot of other factors as well; location, family, friends, enemies, school and maybe even how much the individual knows about football as opposed to a fashionable thing to do.
Without wanting this to sound like 'Alcoholics Anonymous', I'm 47 and support West Ham! I was born and grew up in Orpington, which is in the far north of Kent but is also part of the London Borough of Bromley. The closest professional teams to there are Crystal Palace, Charlton and Millwall but when I was at the age where I 'chose' my team, none of them were even close to being a top division side, and thus were hardly ever on TV, which at the tender age of 5/6 was my only exposure to football. At that time, the clubs seen most on TV from London were Spurs, Chelsea and West Ham. Spurs were almost a Northern club to me

so it was a choice between the other two. As the crow flies West Ham was a lot closer than Chelsea, although I discovered later that public transport is a different thing as I had to travel West (to Victoria) before I could go East on train and tube on a match day. The deciding factors for me (and I suspect other impressionable boys of the same era) was the 1966 World Cup with Moore, Hurst and Peters, plus how the Hammers actually played, along with all the blokes I knew at school supporting Chelsea - I guess that is a trait I have always had, not to go with the majority. So by the age of 6 I had claret and blue blood in my veins, played in a number 9 West Ham shirt for my cub and scout team, got verbally abused most of my time at school for being the only Hammer and have had some of my highest highs and lowest lows while following them. I have never been a season ticket holder nor able to watch them every week live, due to age initially (my first ever game was at Upton Park with my Dad to see Liverpool stuff us 2-0 in 1974, a pattern that has been repeated throughout my support!) and then due to my career in retail which of course meant working every Saturday, but while I was a student in Coventry I went to every away game my finances could stretch to, and of course being glued to the radio on Saturday afternoons and Match Of The Day and The Big Match on Saturday night and Sunday afternoons, as well as reading every scrap of info I could find in the papers. Of course this was when all games were played at 3pm on a Saturday or Wednesday evenings at 7.30pm. Best moment is probably winning the FA Cup against Arsenal in 1980, while there are too many bad moments to pick out one, but in recent memory THAT Steven Gerrard goal in the FA Cup Final 2 years ago; fantastic match though. As Bill Shankly memorably said years ago when asked about West Ham 'Lovely people, lovely club, lovely football - 4 points for us every season' (or words to that effect, in the days when it was 2 points for a win). Liverpool have caused more bad moments than most teams for us, including denying us the possibility of playing in Europe when we finished high enough in the 80's, but the ban on English clubs was in force. Revenge will be ours though (on the pitch of course!)
So, I'd like to hear others' stories on why they support their teams...