The Sun has orbited the galaxy, more than 20 times during its 5 billion year lifetime.

...source

Life seems to have survived whatever the galaxy threw at us during that time. So, the conditions for life are robust, and can survive at any location in the galaxy along the orbit that our Sun followed.

1 lightyear = 9.4605284e+15 meters

Within 50 lightyears there are 2000 stars, of which 133 are similar enough to the Sun to theoretically have terrestrial planets (the rest are tiny brown dwarfs and such). This gives a density of:

1.5707441814703437116308411779923e-46 solar-type stars per cubic meter.... :-P

The volume of a torus is given by:

V = 2*(pi^2)*R*(r^2)

where,
pi = 4*atan(1)
R = large radius
r = small radius

Lets define the volume occupied by the sun as it orbits the center of the galaxy by setting the large radius equal to 30,000 lightyears (2.83815852e+20 meters), and assume that the spatial distribution of stars is accurate out to 50 lightyears radius (4.7302642e+17 meters).

This toroidal volume should therefore contain 196,898,607,802 solar type stars. This number seems high, but the swept volume is huge, so it is correct at least mathematically. The galactic core may have a higher density of stars, but it also has a tiny volume in comparison.

So, we're looking at upwards of 197 billion star systems, like our own, whose conditions for life (or not) are equally persistent. That is, whatever conditions exist in these star systems, they probably survived just as well as ours did throughout their many, many orbits about the galaxy.

Now, I don't know about you, but life exists on earth, so it stands to reason that life exists elsewhere. And considering the swept volume that our own Sun has followed, the "rare" earth hypothesis seems just a touch pessimistic. Dontcha think?