"Can someone explain the term Weekend Warrior and why it appears to be used in derogatory context?"
I was always given to understand that the term 'weekend warrior' really referred to the reservists in the TA, being as that's when they were away on training exercises. Nothing derogatory about that.
Although I also think that it has been hijacked by that certain brand of person with the 'work hard - play hard' view of life. Probably has a very long and strenuous working week, possibly a trader in the golden mile or a sales executive; then spends all weekend rewarding themselves with a major expenditure of adrenalin with such pursuits as paintballing, off-road track driving, base jumping or whatever else fuels their high adrenalin fix. This is a different beast to the former, they probably say the phrase, "would you rather live a long and dull life, or a short and exciting one". They are destined for burn-out and they are to be pitied.
Stupid question anyway, I want a long and exciting life