For me, detachment wasn't about preparing for a separation/divorce. It was about stopping me in my codependent tracks. Just like an alcoholic starts recovery by putting down the drink, I needed to step back from my XAH and disentangle myself from the alcoholic/codependent dance. It saved me. Detachment meant I could take a long hard look at what was happening to me and my marriage without being drawn into the distracting and confusing interactions with my XAH. Staying with XAH turned out not to be an option for me.
But I am sure that there are others out there who manage to stay detached and married to their A's. At my one (and only) AlAnon visit, there was someone there who was a long time member and still with their A. It can be done.
I guess it all comes down to how you want to live your life. My XAH couldn't stand my detaching. Drove him (even more) nuts and his behaviour escalated to dangerous (to me) levels. Staying with him would not have given me the peace I craved. Other A's may not be the abusive, manipulating controlling types and this type of relationship could work with them.