One argument against rail in Hillsborough County you'll hear time and time again, before it is eventually implemented, is that the city and county aren't dense enough for it. 

Mind you, I've never been interested in researching the density question all that much because the same zero density argument was used to dissuade development of Dallas's, Salt Lake City's, Charlotte's, and other light rail systems all over the place.  And yet, despite as much, these systems all succeeded.  Density, it seems, never really turns out to be much of an issue in practice.

"New York City doesn't have the density for a boondoggle like a subways system!  You'll never get people off their carriages and into a train!" - Yankee Anti-Railer, 1905.  See the full sized image.

Therefore, I'm not surprised to find this picture of the 7 Line construction into Queens, New York City, circa circa 1917.  I mean, folks, talk about lack of density!  This elaborate "El-style" extension of the New York City subway goes on for miles into, well, nothing.  Note that I'm living slightly right of the frame in this picture today.  Sunnyside Queens, specifically.  As a resident of that field of dirt above, I can assure anyone, the neighborhood is nothing like this today.

What fixed guideway lines like subways or light rail do is promise something called transit-oriented development.  This extension of the budding NYC MTA, which includes a supplemental road running alongside (now Queens Boulevard), is all about shaping land use and promoting a more efficient use of virgin development soil.  I am sure there are vast corridors between the counties and towns of TBARTA's scope that could benefit from this sort of back-boning early on.  What you think of as Hillsborough and Pinellas County today may be vastly different tomorrow, thanks to a committed mass transit framework.