I didn't want to derail the 48 month extension letter thread but I don't want to move this to the N-400 forum as the people that would answer this question would be reading here and wouldn't be reading there.
Of course every person's situation is different but why would a US immigrant NOT want to file for citizenship? I'm truly curious. Other than those that simply can't hold dual citizenship I can see several advantages above being a LPR. I'm curious what people see as disadvantages.
At least for me personally, I see several advantages:
No more green card renewals which saves money and paperwork. Can't forget to file which could creep up on you on a 10 year card as it's such a long time.
Spouse would potentially get more SS benefits as a widow than working and earning the beneficiary's own. In my case my wife would never reach my level of benefit even when it is 50% of mine as I'm capped at the SS max benefit. She won't even work 40 quarters to get any on her own if we move back to her country, which is probably going to happen before 10 years.
Potentially getting a more powerful passport. This might not be the case coming from Europe but coming from SE Asia my wife's passport is weak trying to get to many countries but the benefit of visa-free travel to many SE Asia countries so she could use hers sometime and US sometime. But if you hold dual citizenship this may not be a benefit but wouldn't be a negative.
No risk of loosing a green card if you spend most of your time outside the US, again, as we plan to do in the future. To be able to come back any time you want even after a multi-year stay outside the US is reassuring.
Again, for me, zero disadvantages because my wife will also keep her VN passport and citizenship. We would still have to file iincome taxes in the US based on worldwide income anyway so, while it isn't a benefit, it isn't a negative either.