It works on Regen
All champions have HP regen. Some more than others, but unless you're Mundo or Heimer it's not worth it.
on heals from allies
Heavily conditional. You need an ally that heals others on your team, and you need that ally to actually heal you instead of the carry. Both of those are completely out of your control. Technically it works on those HP restoring plants that are in every game, but I wouldn't buy it just for that since it's RNG if and where they spawn.
and it's more slot efficient than Banshee.
Tank items are generally cheap enough that you don't need to worry as much about efficiency as you would about AP or AD items. Squeezing the most out of your gold isn't as important when you're not dropping huge amounts on an Infinity Edge or Deathcap. However, Spirit Visage is quite a chunk of cash, so if you can't make the passive do a lot of work it becomes the exception to the rule.
Spirit Visage is more for juggernauts, bruisers, and fighters who have built in sustain or purchase lifesteal items. Riot increased the HP and lowered the resist last season specifically to make it more appealing to those three classes and to differentiate it from Banshee Veil which was balanced for tanks to use instead.
You could argue that since the tank update hasn't come yet that some tanks don't have the higher HP values they need to make the most of resist focused items like BV, but that's part of the reason why tanks aren't doing so hot; some of them get punished for building items balanced specifically for them. That's a huge reason why having the tank update take place so long after the juggernaut update was a horrible idea. The tank update should have happened right after juggernauts, since both of them are half of the whole defensive update.
And MR is harder to get than Armor because Magic DPS is also much more rare than Physical DPS. To resist Magic Burst, HP work just as well as MR, and it's not common as a Tank to die because of a lack of MR, unless you're against a fed Ryze or Cassio.
If this was two years ago, I'd agree with you. However, masteries provide so much more magic damage that it's becoming more and more common to receive constant magic damage. Not only do you have keystones that deal magic damage like Thunderlords, DFT, and Grasp, you have that new mastery Greenfathers that not only deals magic damage on a fairly low CD but also does %HP damage as well. Sure you have to duck into a bush first, but bushes are all over the Rift and multiple are in each lane.
On top of that there's several masteries that boost the damage of what type you're already doing, like Sorcery and all three tier 4 masteries in the ferocity tree. Those all increase magic damage, including abilities that are multi-hit or DoT and chewed through tanks before the boost.
This rhetoric about MR needing to be harder to get because magic damage mostly comes in bursts is becoming less and less true as League evolves. Even building lots of HP is losing it's appeal since Riot seems to be insisting they put %HP damage everywhere nowadays. I remember back when having %HP damage was special; that's not the case anymore.
Make no mistake, tanks need help. They need help now, they needed it last season, and depending on whether or not they're the next class that gets chosen for the mid year update they'll need help for at least six more months. I guess we did get Knight's Vow though, and it'll be fun playing with that. Too bad you can't get two champions to make a vow with each other, since a juggernaut and tank linked as such might actually survive longer than three or four seconds in a teamfight.
A man can dream.
and maybe 
there's not really any options left open. 
is quite expensive (as far as tank items go), only gives the same amount of MR as the portal does, and has a passive that a lot of tanks can't fully utilize because they don't have sustain. Visage is more of a juggernaut item now.
has 60 MR and 30 Armor. I wouldn't be surprised to see jungle and top lane tanks picking it up second or third against AP heavy teams just for the stats alone. That active can still be useful on a non-support as well.
worth of gold. That's 10 
.
is better at splitpushing since it can take down entire towers down by itself if placed right, can hold lanes while you get objectives elsewhere, gets you more gold, gives better tank stats overall, and gives you some movespeed around turrets
is better for building to screw over any ap lane oponent. They took 
top? Wish him luck with clearing that siege minion with his autos. It screws over people's wave pushing power and keeps your turret safe in laning phase. It is also usefull for split pushing, but not as good as zz'rot.
: The Tri-Lane Push is strengthened.
