There are two reasons to mine.
1.) As a for-profit venture.
2.) The add some to, and support, the Bitcoin network.
For #1 - If you pay your own electric bill from a utility company, it's not worth it. CPU mining is obsolete in terms of speed, and they're rather bad at it. GPUs are better, but already borderline obsolete due to FPGAs. And "very soon (tm)" they WILL be obsolete, with the release of ASIC devices in the next 3 weeks to 3 months. If you want to be involved in the Bitcoin community, purchasing Bitcoins is the best bet for profit. The fees take less out of your pocket than electricity would, and that assumes you already have the hardware.
Note: If you do not pay for your own electricity, this might be inaccurate for GPU mining. Some governments subsidize electric costs, and some people have non-utility power (solar, hydro-electric, et cetera). If this either of these is the case for you, mining MIGHT be a profitable option.
For #2 - DO IT! One of BTC's strengths is that it is decentralized and peer-to-peer. One real (but diminishing in importance every day) risk to the Bitcoin network is a 51% attack - where a single person or group has 51% or greater processing power on the Bitcoin network. Every little bit of hashing power helps offset this risk, and the more individuals that do it, the better. But it's not a profitable venture. It just adds to the nebulous concept of "a stronger Bitcoin network".