personally, I enjoy the MS build on fights like Nef and Chromag (even chain pulling trash). having 10.1k mana without distilled is such a nice pillow to have.
my healing doesn't really change without SH specced. my FH's lose maybe 60 healing on avg for r5. I still touch over 1,000 r5 FH without SH specced.
I don't know, but personally I like the mana scaling better over some fluff healing added to an already strong heal.
also save on mana pots too

MS gives benefits outside of just healing power which I like a lot. (cleansing duration, healing duration etc.)
MS might become more appealing when fights become longer and you start regretting having that 60 more healing to your spells over missing out on 1100 free mana.
holy nova is very useful. while moving, you can use it to instant heal yourself and party members. it is by far the only instant cast party heal if you're grouped up. even for soloing, I have seen max rank HN crits for 1000 before. it is pretty huge on the healing part. (heal while being mobile is nice to have)
for instance, if shazzrah teleports on top of you. do you just stand there and eat the arcane explosions? risk a counterspell during a channel? running away from shazzrah and using holy nova keeps you alive AND prevents a CS lockout on holy spells.
you don't see many fights for HN, but having it can be huge when you're running out of stuff. (Naxx and AQ will see more use for it)
plus you can solo the plants in DM:E with it too.
