Since I've seen this thrown around a lot, I'm going to put in my two cents.
First of all, let me give you a similar situation for you to consider.
A child isn't born knowing how to speak. She learns from hearing her parents' voices and parroting the words. In the beginning, she will only copy the words. However, as she ages and she becomes more familiar with the words, she will construct her own sentences. But her vocabulary is narrow. She will learn new words from other people. Some are bad, some are not. Some are used frequently, some are not.
Initially, she will speak just like her parents. But as she ages and becomes even more proficient with speech and begins to form her own opinions, her speech pattern will start to differ from her parents' speech pattern and incorporate elements from other people's speech patterns. However, at this stage, mimicking is seen to be extremely rude unless she is directly quoting someone.
This is a similar situation to referencing art (NOT TRACING). At first, while someone is developing their style, they will reference others' art. But as they become more confident and more proficient, they will adopt other elements from other art styles, and make some elements completely their own. This is normal. But once someone fully embraces their own style, mimicking others' art styles is seen as rude unless given permission.
As you can see, there is a place and time for referencing, but it is only in the beginning of development or in particular circumstances.
Tracing, however, is unacceptable unless given express permission by the artist in question.
