Mining in your work/home computer has some side effects, namely:
- Heat - could damage your hardware if you don't handle it properly
- Slowdowns - if you use your computer while you mine you should reduce the miner's load
- Power consumption - if you don't pay for electricity this may not be an issue
So I would say that there is a point in casual mining as long as your earnings compensate for these disadvantages. It is really up to you to decide if you want to stress your hardware to earn a few coins.
Speaking of a more concrete scenario, casual miners usually fall into two categories:
- You have a good GPU for mining in your PC - a recent medium/high end ATI card.
- You don't have a good GPU so you want to mine with your CPU/low end GPU.
Check this question to find out where do you stand. If you have a good GPU, you might get around 8 bitcoins per month at current difficulty. If you don't, you will probably not earn more than 1 or 2 coins per month.
My personal opinion is that if you fit into the first category, you could give it a shot. If not, I wouldn't do it just for a few dollars (and I do not recommend mining with your CPU at all).
As a final note, bear in mind that these numbers are always changing: new hardware is coming out all the time and new miners are joining while others are leaving. This influences the amount of bitcoins you will earn with your hardware so be sure to check an updated hardware list for your calculations.