They've already explicitly stated their design intent is for Warlocks to (ideally) only need 3 shards per encounter.
It's their intent to be that way. If it wasn't, you could use them in combat. It's not poor design, it's different design.
They want it to be like bloodlust. You use it at the right time, not all the time.
Yes, this is the design. Outside of combat (which means between encounters, not when you can get out of combat for 2 sec in an Arena), you can get the shards back quickly from killing things or using Harvest Soul if there is nothing conveniently around to kill. We don't want Harvest Soul to be super convenient. It's your backup option.
If we find that it's super frustrating as a lock in a long encounter (including a long Arena match), then we'll look at ways to get shards back in combat. However our prediction is as soon as we do that, locks will start blowing their shards on cooldown and then getting frustrated at a slow Aspect of the Viper-like mechanic to get them back.