Okay okay. Sounds crazy. But recently, I've been experimenting with Sigil of Purity over Sigil of Cleansing on my melle set, and it is amazing. The trick here is that sigil effects from your current weapon will stick with you while in shroud form. So you can in addition to cleansing a condition, cleanse an extra condition every 4 seconds, while Dealing damage in shroud. If you use Death Magic like me, along with shrouded removal, you are removing 2 conditions every 3 seconds, AND 2 conditions every 4 seconds. The sigil will only proc when you have a condition on you, so it will activate when you NEED it too (which is great). You become an anti-condi god and you basically laugh at any scourge or condi-mesmer that thinks you are an ez target. Another reason this sigil is better than cleansing in some scenario's, is because conditions will tend to deal their damage over time, and thus the longer these conditions remain on you, the more value they are attaining for the enemy. Cleansing sigils will force you to weapon swap, and will cleanse conditions at intervals associated with your ability to weapon swap , which means you might have to keep a torment or bleeding stack(s) on you longer than it needs to be. With Purity, they get removed as soon as possible, keeping you clear of conditions as much as possible and thus reducing the value of any condition stacking. In a way, Purity is a sustained condi-removal tool (so if you are fighting a class that tries to pressure you with conditions, this will counter that) and Cleansing is a burst condi-removal tool.
To take full advantage of this sigil, you need to try and meet the following 3 criteria
1) Play dagger
2) Use Death Magic (Shrouded Removal)
3) Use Anti-Toxin Rune
Don't be scared about the Flanking requirment on the Sigil. Flanking is a 270degree window of opportunity on the enemy character, and in shroud as long as you pass through your enemy while dealing damage, you will proc this sigil when it is needed, and you only need to have one damage packet land, while you are positioned in this 270 degree window.
You don't HAVE to play dagger, it's just better at dealing damage in smaller packets that can trigger the sigil more often. With Greatsword, autoattack isn't even a logical function on that weapon and so you have less opportunities for damage packets to go through your opponent, and thus you won't be able to trigger the sigil as often as you would like.
Personally i play this build now (One Sigil of Purity on my Melle weapon, and One Sigil of Cleansing on my ranged set.)