Yeah, it would be nice to be able to force a particular deity to spawn - if only for beta testing. (That's actually something I'd appreciate for items and glyphs, too...)
Earthmother grants a free IMAWAL if she's your initial deity. Converting from another deity will give you enough piety for Plantation, which is a lot better than it was in the alpha (there are more ways to remove or avoid plants, and as Darvin noted she only punishes for killing mundane plants), and it was already pretty decent in the alpha. The anti-poison boon seems fairly useless to me, although I suppose it makes it easier to clear a path through all the plants she creates; perhaps it should also remove one layer of corrosion. The plant removal boon is probably necessary, although I keep forgetting to manually clear out as many magical plants as possible, first (if it's possible to kill them without harm, it's a waste to let the boon remove them). On the other hand, Earthmother's punishment is another that can be safely ignored in certain circumstances (specifically, if you're relying exclusively on CYDSTEPP, BURNDAYARZ, or PISORF).
My one Earthmother run was with a CYDSTEPP Elf Rogue, so I mainly tried out the conversion point boon; it's decent, probably a lot better with an Orc or maybe a Dwarf, but seems a bit underpowered to me (an Elf can get MP far more efficiently from Dracul or even Mystera). I can see the shield boon being very strong with the right class and glyphs, particularly since that form of damage reduction is unaffected by enemies that reduce resistances, but I'll wait until I actually get Earthmother with the right class to test that theory.