Stemmer or dictionary?
There is another reason: words which don't appear in the dictionary. Of course a dictionary approach will correctly stem all the forms which are known in the dictionary, and depending on the language this may indeed lead to better accuracy. However the dictionary approach cannot do anything about unknown words, whereas a generic stemmer can try to apply its generic rules. This can be particularly important with texts which are either very domain-specific (e.g. medicine), which often contain technical words which are not in a general dictionary, or recent user-generated texts such as social media posts where people may use neologisms or words borrowed and sometimes transformed from another language.