Centralization of control
As history shows, small miners form big groups to reduce variation of the reward. This leads to concentration of power (computational and political) in hands of few pool operators and gives them ability to apply wide spectrum of policies (filtering, postponing) on certain transactions. Although there are no known cases where pool operators abused their power, there have been several instances where the opportunity were present. This possibility in a monetary system powering a multibillion (in USD) industry is completely unacceptable.

PoW with no block reward [to avoid centralization]. [...]

[...] Or IOTA owners have to "hire" miners. [...]