Countries that do not allow dual citizenship is a good reason, and maybe the cost and paperwork required by other countries (although, unless you really cannot afford it its worth getting it to cover yourself in case rules change). I did it myself.

The same goes for the cost of British citizenship. once you have it its just the cost of renewing passports (about £100/decade at current rates) for the rest of your life. That £2,000 is much less than the cost of renewing some types or residence visa and gives you the security of being a citizen.