It is important to remember that until your marriage has been dissolved by a court of competent jurisdiction, you are still married. For probate purposes, this means that your spouse- no matter how long you've been separated- still stands to inherit the majority of your estate unless you create an Estate Plan stating otherwise.
Additionally, as the legal next of kin, your spouse is the person healthcare workers and medical professionals will look to when making decisions about your healthcare. Decisions such as whether or not to maintain you on life support, or employ other life-sustaining measures could be in the hands of your estranged spouse.
Finally, living separate and apart from your estranged spouse, regardless the length of the separation, does not dissolve your marriage. While most jurisdictions have provisions that may make it possible to defeat an estranged spouse's claim, this is often a lengthy and expensive process.