But army summons are also not always better.
Dwarves are known for their strong buildings and for having some of the best architects.
A troup summon doesn't really show something special about the dwarves, it just gives you some units, that's all.
While Isengard is known for building a huge army within a small time (therefore they can summon a big army), the dwarves don't have the possibility to create such a huge army within seconds and are more known for mining, building some fortresses and stay there.
This is perfectly shown with the last spell.
Why remove something which shows one of the most important aspects of a faction and remove it with a generic spell which just shows one event where the faction participated in?
This spell will most likely be used very often offensive and I personally don't think that it fits for the dwarven to have so many strong offensive possibilities, while they were in truth far more defensive in the third age.