Let's take a quick look at the Eagle, shall we?
Essentially, if you are going to fit rails you are pretty much limiting yourself to fighting at extreme sniper ranges. Here's a fit I came up with:
Yes, you could go for uber range sniping with sensor boosters and 250mm rails with spike ammo but that is such a niche application with limited PvP opportunities that its almost not worth considering.
Let's look at a blaster fit Eagle. Perhaps the optimal range bonuses makes the blasters on this ship more flexible and dangerous?
Compare this with a shield (!) Deimos:
So to sum up, the Eagle's advantage of big tank in short range combat is negated by its slowness and anemic damage, and negated in long range combat by its extremely long range capabilities. The only niche it fits in , the Eagle's Nest if you will, is ignoring its tank and going full out snipe fit and hoping things will move slow enough or get tackled enough so you can hit them.
If you really want blasters on a Caldari cruiser, save yourself the ISK and buy a few Moas instead: