For a long time I have used them as a move to space, so a Farming Opportunity near Park Bench took you past the Salary square. I didn't go as far as allowing people to play cards and not enter a career, this would clearly be cheating?! The other obvious interpretation is you move directly across the board paying not attention to any spaces you are near or 'pass'. Again you could maybe argue that you could use them and not partake in the career, (The obvious exception for this is Vacation which you could legally leave next turn anyway.)
I have the 2003 version. Rules are fuzzy, but I do recall that if you use an Opp card, you MUST enter that career path/loop.
For me, the reasons to use Opp cards include: 1) guarantee-ing that you'll be able to enter a career path/loop. There's always Experience cards to help you out, but this way, you can get there quicker, and save the Exp cards for later where Opp. cards won't be able to mitigate luck on rolling
2) get in a career loop for free. Few of them do say 'all expenses paid'.
3) pass start to collect an easy hit of salary
4) bump people. Someone getting into teaching to collect what you suspect is more happiness points FTW? Play your Opp card that takes you to teaching. Then you can either bump him, or force a deal such that you can slow him down if he wants to stay there.
5) avoid being bumped. If someone is trailing you on the board, Opp card lets you "teleport" safely far away.