Anti-Magic Shell is a core ability for death knights. It has many uses, able to absorb large amounts of magic damage, to convert that damage into runic power, and to become immune to many magic debuffs. Because of this, it's both a defensive ability, and an offensive one. I'll be primarily going over the offensive function, but I'll not be ignoring the defensive.
One of the most important marks of skill for death knights is AMS timing. I know one of the first things I look at in another dk's logs is their total runic regen from AMS. Converting damage into runic often means using AMS at the right time, before a significant burst of magic damage. Sometimes you cannot afford to do this, on jin'rokh for example, you'll need AMS to up be during the ionization, and soaking runic should be a low priority. On most fights however, whether you're soaking megaera's rampage, or lei shen's thunderstruck, or durumu's eye maze, skilled dks use AMS whenever you'll get the most runic (and therefore damage) absorbed.
But there is a problem. AMS is treated as some sort of secondary absorb, and all other absorbs take priority over AMS. This means that the power-word shield you have on you will prevent AMS from giving you any runic power at all. This isn't that big a problem during major sustained magic damage, as both AMS and outside absorbs are used at there full potential, but in low damage scenarios, you are often robbed of most of the runic from AMS. I've personally been having a problem with this on both twins night phase, and lei shen thunderstucks. Being far enough away that the thunderstruck only hits for 100k, the spirit shell on me makes ams useless, and I've been expecting and preparing for that full runic bar. It's a horrible feeling to expect your runic bar to fill on AMS, and to get nothing.
To solve this problem, I've created a macro to cancel every absorb known to man or orc. This allows AMS to absorb it's full runic potential and should be considered necessary for optimal dk play until the time comes that AMS is calculated before other absorbs.
/cast Anti-Magic Shell
/cancelaura power word:shield
/cancelaura spirit shell
/cancelaura divine aegis
/cancelaura life cocoon
/cancelaura illuminated healing
/cancelaura sacred shield
However, a word of warning and a word of fun. This macro removes all shields and bubbles on you, which can lead to your death. Do not forget that a dead dps does no dps. As such, if you are ever absorbing any magic damage which has even the possibility of killing you through AMS (I use 100% of your health as the point where that becomes a threat), this macro is both unnecessary and dangerous. Only use the macro where the damage you're taking cannot lead to your death.
And for the word of fun, try asking your healers what the names of their absorbs are, to make sure I did not miss any. When they ask why you want to know, tell them you're making a macro to remove all of their absorbs. It's very entertaining.