Is it useful to use both a shield spell and lets say Heavy armor?

For the most part, no. It can be useful early on, though, since the effects stack.

I say yes. I'm currently playing a ronin, who uses almost all heavy armor. The shield spells (actually potions for me) are invaluable when mobbed by three or more combatants. My heavy armor is currently 80, and the shield effects help me out with the inevitable battering I receive from the aforementioned enemies. I almost never use it one-on-one though, unless I am practicing my H2H skills against Minotaur Lords.

Armor rating is capped at 85. So using a shield spell to supplement your armor up to this point can be a help, any more shield is a waste of magicka.

Sort of like what Rhomphaia said, getting an armor rating up to the max of 85 can be helpful for some characters.

You can use any combo of sheild/elemental shield spells/items, light or heavy armor, potions to achieve this. Anything beyond 85 will not register or help you. At higher levels you can reach 85 with simply four properly enchanted items of clothing - so what you wear really boils down to looks at higher levels.

Just want to clarify that the 85 cap is for Armor Rating, and that the added protection from elemental shielding (fire/frost/shock) can go up to 100 (i.e., you can receive 100 versus elemental damage even if your armor rating for other damage caps at 85).

Yes its quite usefull but spells in general tend to act poorly with any armour on. most characters I play are light armour wearing paw to toe Khajiit warrior mages so it helps a fair bit.
Especially as it takes me ages to enchant equipment, it can help keep you alive if you run into trouble, but the best advice I have found is use it as a buffer but never rely on even an 85 armour rating unless you are playing on an easy setting, or are facing just one enemy.
Rather get used to building up an arsenal of tricks to stop you getting hit enmase in the first place imo.

It can be of great help early on when your are desprete for every last point of DR. Kinda fades when the high-end armors open up for you.....but it still allows use of the lesser armor sets late in the game. Some of them look very nice, but come with their own enchantments already.

That's exactly the way I look at it.

Many of my characters, when they're in fur or leather or iron or steel, cast Protect or, if they can, Defend right before every fight. Defend in particular is a huge boost early in the game-- that 15% often more or less doubles their AR.

By the time the later armors start showing up though, they just don't bother any more. By then they not only have much better AR, but they've got much better weapons, so they're generally killing things faster and therefore being hit fewer times anyway.

My melee fighters started augmenting their worn armor with shield spells a long time ago, when I started playing my first two-handed weapon character (Tadrose Helas). I found it was practically a must for low-level fighters, given how much it helped. She only used three spells. One to heal, one for the shield, and one to open locks. The shield got the most use, as she cast it at the beginning of every fight.

Yes, my char uses it currently (a pretty normal armor rated Light Armor, plus two rings, Frost & Shock Shield) in order to reach 85 Total Armor Rating. That's especially useful to complement an armor you like to wear, but that is weak (has low AR).

BTW, try to use always Fire/Frost/Shock Shield, as they combine the effects of Shield plus Resist Fire/Frost/Shock (respectively) all-in-one

At the higher levels, you will almost certainly run into Bandits that have weapons enchanted with 'deteriorate-armor. I can guarantee it in OB gates. That Daedric armor's 85% protection can melt away very rapidly and you can't repair it during a fight. I wouldn't sell off all those shield potions just yet.