It may have been before your time playing (maybe not), but when Deathwing was running the 40K tournaments at Gamers Inn for 4 years, we always had a sportsmanship score. You're right, in a lot of cases everyone maxes out and it becomes a non-factor in the overall scoring/results, but in some cases it was a deciding factor, and by a small margin. We evolved to a 4 point system that didn't weigh as heavily as W/L/D scores, but it could tip the scales in a close event, and I really can't remember a time when someone "abused" it by tanking all their opponents. I had that happen to me once, years ago, but we were on the bottom table and the person who did it didn't surprise me by doing it. Anyway, it can work, and knowing that it's there can tend to make players be on somewhat better behavior than they might otherwise when there isn't a sportsmanship score involved.
On the other hand, I know a lot of players who don't like to lose a tournament based on a subjective score when they otherwise were undefeated. It's a tough balancing act to get it right, and depends on your player community, which does tend to evolve over time. Deathwing no longer runs tournaments, because we don't need to, seeing as Jimmy runs a 40K event twice a month at Empire. But I think sportsmanship is an important part of any game, and you get out of it what you put into it.
A bad day of gaming beats a good day of work (or school) every time!