This is not OK. Here's why:
1) Nobody else is bad at 'stack healing.'
In order for the 'strengths/weaknesses' model of class balance to work, strengths and weaknesses have to be set up to be complementary. One class's strength balances another class's weakness; if you bring complementary classes, then your team as a whole is pretty good at everything.
But nobody's weak at stack healing. The only class that even arguably might get a little weaker when the raid is grouped up and stationary is priests (due to Cascade/Halo), and that difference is pretty minor (and arguably outweighed by Power Word: Barrier for Disc). Everyone else gets stronger when people stack up.
What's more, short of redesigning another class's raid heals so they all work like Cascade/Halo, you can't even make another class weak at stack healing. So there's no complement for shamans, and there isn't going to be.
When you bring a shaman and any other class, instead of having a strong stack healer/weak spread healer paired with a strong spread healer/weak stack healer (balanced and average in any situation), what you have is a strong stack healer/weak spread healer paired with an average stack healer/average spread healer: above average and potentially OP in a stack, below average and potentially not good enough in a spread.
2) Stacking and spreading are mutually-exclusive.
Most strength/weakness pairs involve things that might happen at the same time. You can have single-target and AoE damage at the same time (every fight except Baleroc). You can have sustained damage and burst at the same time (e.g. Garalon, any council fight). You can have scattered/triage healing and raidwide AoE at the same time (most fights). Healers can play to their own strengths most of the time, complementing each other nicely.
But spreading/stacking is binary. Either the raid is spread out and moving, or it is stacked. In 10m, there is absolutely no blurring of those boundaries. In 25s, you might have the melee stacked while the ranged spreads out, but there are a number of problems with just healing the melee. Notably: Everyone else's raidhealing abilities are healing them too, while yours aren't hitting the ranged. And since Healing Rain scales with the number of targets it hits, it's not strong enough on the melee alone to compensate for your inability to heal the ranged.
3) Many well-designed and fun encounters do not include any sustained stacking.
It's fairly rare to see encounters with no single-target healing or no AoE healing or no burst healing or no sustained healing or no random damage or no raidwide AoE. These sorts of encounters are usually rather unique and widely understood as gimmick fights. They're typically not a big deal. One healing class or another might struggle on them, but they're still a valuable enough asset to the team on other encounters that the team is motivated to find a way to make it work.
On the other hand, there are plenty of encounters where the raid needs to be spread out and/or mobile the entire time. These are ordinary and common and don't feel at all gimmicky. "Moving out of fire" is a core WoW mechanic, one of the two primary ways the encounter designers use to make fights interesting for DPS (the other being target-switching). Mechanics-focused tiers, like Throne of Thunder, tend to involve a lot of fire. When a healer can't deal effectively with a raid that's moving out of fire, they're at a pretty serious disadvantage.