I think the main problem with sanctification is most people assume you need to make all 4 of your teammates invulnerable for 3 seconds soaking 1,000+ points of damage or otherwise it is useless, but I want to provide some new ways of thinking about this ultimate.
- It is an AOE cleanse - by making your team invulnerable, you removes all stuns, roots and slows, great to quickly save one or all of your team from - jaina's chill, etc's moshpit, murky's net, any stun from their warriors (Etc, muradin, Joanna, Diablo), it can remove living bombs from a flamestrike, remove envenom, Butchers brand or lamb to Slaughter and it can be used as a quick disengage tool to allow your team full move speed again. You are not worried about being stunned in this case as the effect is immediate and will persist even if you are stunned. You can also Press R again quickly to cancel channeling after the initial cleanse effect is achieved and get back into the battle.
- It is an AOE Iceblock - many heroes use Iceblock to quickly dodge their opponent's burst damage and waste their cooldowns - Brightwing, Malfurion and Jaina usually take this ability at 13 as they lack escape tools. Some great ultimates to dodge are Triple tap, Precision strike and Pyroblast. Sanctification is an AOE Iceblock for your entire team. Wait until the last second to use this, so you can't be stunned out of it as you are just trying to mitigate the burst in this case to keep your teammate alive. Any damage mitigated after the burst is just gravy. All of these ultimates have much longer cooldowns than yours, so even if you can trade only 1 for 1 in ultimates, it should be considered a win.
- It is a great counter to dive comps or melee or single target assassins - Kerrigan, Illidan, Zeratul, Nova, Butcher, Sonya, Thrall. These assassins need to go deep, engage and trade with your ranged damage dealers or healers to do their job, and most lack stuns to cancel your ultimate. When these assassins jump on your team, use your ultimate. This both removes any slows from your teammates and blocks any damage, allowing your allies to stand their ground or disengage if they want. Except for Zeratul, most of these assassins do not have good disengage tools themselves, so once they jump in and use their burst, they are stuck. In addition, Sonya, Illidan, Thrall, The Butcher and Kerrigan rely on doing damage for their sustain, if they are not doing damage, they are not generating health/shields and are much easier to kill. If your allies survive their initial burst, these enemies can be focused down. Again, just an example of how this can be used - if you hear the Butcher say "Fresh Meat" on one of your allies, get close to them with your Q, wait till the last minute and press R - your Sanctification will cancel lamb to the slaughter, remove Butchers Brand and cancel the stun on Butchers E. His Furnace blast will go off for no damage, except to yourself and he will now be stuck in the middle of your team as an easy kill and his only stun is now on a 20 second cooldown. You have completely countered all of his abilities.
- It can be used for simple damage mitigation during a chaotic battle - cancelling 3 seconds of damage for even 1 or 2 members of your team can easily swing a battle in your team's favour. Even if your team doesn't take full advantage by standing still and maximizing damage, you still are providing a very powerful shield to your team
- You can spam this ability as the cooldown for this ultimate is only 45 seconds. For comparison, single target Iceblock has a cooldown of 60 seconds and and single target cleanse has a cooldown of 30 seconds. Given that the channeling time on this is 3 seconds and that you will most likely take Battle Momentum at level 7 to reduce your cooldowns - you can have this ability up every 30-35 seconds or so, which means it should be up for every team fight and you should never feel bad about using it.
To make the most of the above, it requires a bit of a change in your mindset, you are no longer the initiation tank looking for when to jump their back line, but almost a tank/support/bodyguard, looking for when you can use your ultimate as the perfect peel for your backline.
Those are my two cents, please give it a try this week.