I would think you'd want to formalize the divorce at some point, to avoid the akward 'I can't marry you right now, cause I'm sort of already married' stuff, especially if you and your legal spouse move away from each other and fall out of touch.

But, if you're in the US and you're otherwise happy to be legally married but separated, you may want to consider waiting to legally divorce until you've been legally married 10 years. That way, either can use the other's social security record to claim as a spouse if unmarried (at no cost to the other ex-spouse).