Well, we have had a few people on the forum who have left the UK after getting ILR but without getting citizenship... And they haven't intended ever moving back to the UK.
However, a few years later, their plans changed and they decided to move back to the UK... But their ILR expired after 2 years away and because they didn't get citizenship, they had to go through the entire spousal visa process all over again.
I think they all regretted not getting citizenship before they left the UK - it would have saved them 5 years of visas and thousands in visa fees.
Also, UK citizenship allows you to visit countries that you would need a visa for with a US passport, and countries that you can't enter with a US passport (I.e. Cuba).
Personally, if I had the opportunity to qualify for citizenship of another country, I would jump at the chance to have it (perhaps not the US though due to the tax issues

).
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