What use do Skyshards have

Right now there are three NPC guilds in the game, which you can join - all offering a unique skill line for your character:

• The Fighters Guild is pretty much what you expect them to be. They've taken a massive contract to fight back Molag Bal's Dark Anchors and all Daedra.
• The Mages Guild is pretty much what you expect them to be as well. They're trying to preserve knowledge throughout the times of war.
• The Undaunted are new in the series. They're a group of adventurers trying to prove their worth and they're a bit like a group of posers. They want to fight the biggest, strongest, meanest creatures and win.

Unfortunately, some guilds didn't make it right into launch , but they're scheduled to be added at a later date, most likely later this year:

• The Dark Brotherhood will be added later. They're still quite the same as in the previous games and will most likely offer contracts and bounties.
• The Thieves Guild will be back as well. They didn't release any details so far, but it's quite likely that its introduction will also introduce a crime/stealing system.

Also, there are quite a few other groups in the game. Some you'll be able to join, but they won't necessarily grant you direct bonuses or skills. Warning: spoiler ahead!

• The Eyes of the Queen are a network of spies working outside the Aldmeri Dominion's laws (as far as necessary) and subject only to Queen Ayrenn herself.
• The Morag Tong are quite similar to the Dark Brotherhood, although they're quite focused on Morrowind.
• The Shadowscales are a group of Argonians bringing the gift of Sithis to those worthy. Another group of assassins (is there some pattern? ;)).
• The Beldama Wyrd are a circle of witches from Glenumbra. They're pretty much classic druids, but only women.
• The Companions you've met in Skyrim appear in ESO as well, but you're not able to become one of them.
• And probably many others I just forgot to mention right now... this list is definitely incomplete.

Of course. All effects should be affected by soft caps (unless stated otherwise; so it's less effective since you're already at the cap).

I think this variant is a lot more interesting if you're not relying on heavy armor (i.e. if you're wearing a possible mix or maybe even only light/medium armor).