u can mass turrets easily with a few scv, which makes turrets better than spore colonies realisitcally speaking. much more superior AA on an island map as well since you can drop a few scv's and you have instant AA def on an expansion island. spore colonies.... you gotta make drones which takes away larvae from making something else. not really critical unless you care about making something else
and what you said bout goliaths, although they are much better for anti-air, turrets are still only 75 mins and provide solid AA protection for ur base. do you really plan on keeping ur goliaths on guard duty instead of with ur assault force as protection against air? i dont think so.
plus, i love being able to make a wall of turrets two or three deep in my base
(even making one line of turrets is easier than making a line of spore colonies) you cant do that with spore colonies unless you got a lot of time on ur hands to make all those drones and ur opponent (s) aren't doing anything.
now if you do a side by side comparison of a spore colony and a turret, spore colonies come out on top, but i don't think two sole AA buildings are going to be duking it out anytime soon... you gotta look at the big picture and missle turrets are better.
oh and turrets can be replaced easily by pulling an scv from somewhere, unlike a drone which must be made, thus taking time (or if you took a drone from ur min line, u gotta make another one to replace it)