Well, I'm not really trying to contradict the valk's value... I'm simply stating my preference. for instance, I'd choose goliaths against carriers because I usually have multiple factories... instead of building more starports, I'd use the factories to save time. Even if I were to kill them with valks, I couldn't hurt goons, whereas goliaths can.
About island maps, I'm not really used to using valks offensively... please explain to me how you use would valks on island maps...
I'd use goliaths on island map because in case the drop does succeed with maybe one destroyed, I can attack the ground units whereas valks cannot in case some are managed to be dropped...
If I had a surplus of money, I should go for valks, which I should get used to... normally i go for wraiths if i had a surplus though...for cloaking and land attacking too...
About valks not having much hp or armor: this is true i guess if you compare it with the cost... but don't do that... during the middle or late game, you'll probably have a surplus and the cost probably won't even matter.. at least to me it usually doesn't...
in the end, it's not the cost that makes this unit better than the other, because you may have sufficient funds, but rather how strong the unit is. If you evaluate the unit by comparing it with the cost, that's pretty much assuming that both players will get the same amount of money... If I were to go for valks, wouldn't i try to get tons of money first? and if i were to succeed, it's not a matter of how much they cost, but whether you have them or not...
You can get goliaths and have like tons of money left, or get valks with not as much money left... yet valks have more armor and hp (i think armor maybe)...
in a nutshell: don't compare units to costs during middle or late game... I'm really tired and sleepy right now so you might not understand much...lol