I think the thing about the Myconids is that with a bunch of them together (especially with a Rotpriest), they have a lot of choice as to when each of them dies. So the Rotpriest can start out by wading into melee ahead of the Guards and use decomposing spray for a few turns -- then when the accumulated damage starts to add up, it can fall back to the other 'nids before deliberately sacrificing itself for a Life Burst.
Shift-giving doesn't seem like much, but it's pretty nice for setting up flanks and getting the most out of the other creatures' bursts. I sort of see the Sovereign as mostly standing a few squares back, occasionally darting in to blast Spore Burst when it recharges. It has good AC and Fort, though, so it's not really bad at all in close combat.
"They have a lot of choice as to when each of them dies."
This gets to the core of the matter with Myconids, particularly in light of what we read in the
Player's Handbook combat rules with respect to rendering creatures unconscious. [1]
Reducing a creature to zero hit points—or less—does not necessarily kill the creature. It may only render them unconscious, at the discretion of the character using the power or ability that did the damage.
In the case of a rotpriest using Sacrifice for the Colony, that woulld be
itself, or any other Myconid using Roots of the Colony, i.e., they can choose to render each other unconscious, rather than killing one another.
Since the rotpriest healing power triggers on being reduced to 0 hit points—not dying, but being reduced to 0—the rotpriest can thus remain alive, but unconscious. If there are two or more rotpriests in the encounter, they can, in theory, "see-saw" each other, with the healing burst of one bringing another back to consciousness, albeit with only ten hit points.
A party of adventurers could even "kill" a colony, with most of the damage being distributed through Roots of the Colony, only to discover—after a short rest—that the majority of Myconids were only rendered unconscious, not dead.
—Siran Dunmorgan
[1] I believe it's on page 295, but I'm relying on memory, and could be incorrect as to the page number: I am certain about the rule itself, however.