dual citizens?

That entirely depends on where I am going and what passports I use.

If I try to enter the USA using a non US Passport the first thing they are going to do is scan my fingerprints. That is what they do with all non US citizens when they enter the USA.

Immediately the computer would blip and then show I am a US citizen. Seconds after that I would be escorted to a small room and interrogated for three hours by a bunch of Homeland Security Goons.

If I would have another passport on me when I enter the USA and they find it, again I will be talking to HS or Interpol for hours on end,

There is nothing illegal about having multiple passports in the USA. But when entering the USA, US citizens must leave and arrive with their US passport in hand.

Now there are times you would want to carry all your Passports with you but that is only in extreme situations.

If you give me some ideas of the passports and where you are going, I might be able to give some insight.

This is a huge issue with many variables.

Of course I always enter the USA with my US passport, and Iceland with my Icelandic passport. I'm talking about traveling elsewhere, like if I'm only in Europe, or if I were to go to Asia.

I've considered that there of course some places where having the possible power of the US embassy would be useful so if I were to ever to travel somewhere politically unstable I'd go USA. But in Europe where I can go visa free as long as I like on IS passport, is there a logistical or legal reason it's a good idea to always have both?

That is a good question.

I think it depends which country/countries you want to visit in Europe or Asia. If both passports are visa free why carry both of them with you. On the other hand if one of them requires to pay visa fees then you can use the other passport.

But you said it before: if you leave Iceland with your Icelandic passport then you have to enter Iceland with the same passport and vice versa. Then your question is been answered by yourself. 

It is legally and sometimes logistical to use them both on your journeys.

It really depends on the destination country and their international agreements regarding visas or visa waivers as to which passport you would wish to use. For example, since Iceland is one of the Schengen Area countries you would probably choose to use that passport while travelling anywhere in the European Union or in other Visa Waiver Program (VWP)countries. For example presently if you wished to visit Brazil having only a US passport you would need to apply (in the US) for a VITUR Tourist Visa before you could visit, however with your Icelandic passport you don't need a visa, you just present your passport on arrival for a visa entry stamp and again on departure for the exit stamp.

There are also countries that would be much safer for you to enter if you were using ANY other passport than one from the USA. (Wonder why?)

Conversely, you will also find that entry into the USA can be much more difficult for someone holding a passport from many countries. Someone arriving on a French passport is certainly not going to have the hassles that someone arriving on a Iraq passport for example.

The only real consideration is that most nations have citizenship laws that, while they recognize dual/multiple citizenship, are also quite specific about the fact that while in that country regardless of any secondary citizenship, if you hold citizenship of that nation you are considered, first-and-foremost to be a citizen of that nation with all the rights and responsibilities that apply, so one can't fall back on any secondary citizenship if something goes wrong and it all hits the fan.

  Cheers, 
  William James Woodward – Brazil Animator, Expat-blog Team

I always travel with both my passports, NZ and Australian.

I would always take both.  You never know what's going to happen.