You don't need a triathlon bike to do triathlons. Modified road bikes are
very
common in triathlons. If you already own a road bike or plan on doing other
types of riding you may be better off with a road bike with clamp on
aerobars.
The advantages of a triathlon bike are in the posisitioning. They are setup
to
keep you more comfortable when in the aero bars and to work the quads less,
saving them for the run. Often triathlon bikes are more aerodynamic than
road
bikes.