Here's why:
* Traditional Grammar: Traditional grammar dictates that "they" is a plural pronoun. Using it as a singular pronoun can be seen as grammatically incorrect by some.
* Ambiguity: When "they" is used to refer to a singular person, it can be unclear exactly who is being referred to. For example, "Someone left their phone on the table" – it's unclear who the "they" refers to.
* Gender Neutrality: "They" is increasingly used as a singular pronoun to refer to someone whose gender identity is non-binary or unknown. While this usage is becoming more common, it can still be seen as confusing or incorrect by some.
Example:
* Ambiguous: "The person in the corner left their coat behind." (Who is "they" referring to?)
* Clear: "The person in the corner left their coat behind." (We know the gender of the person in the corner, making "they" unambiguous.)
Alternatives:
* Use a noun phrase: Instead of "they," use a more specific noun phrase to avoid ambiguity. For example, "The customer left their phone on the table." -> "The customer with the red shirt left their phone on the table."
* Use "he or she" or "he/she": This option is less common nowadays, but it can be used to avoid ambiguity when referring to a person whose gender is unknown.
* Use "they" for gender-neutral language: When referring to a person whose gender is unknown or unspecified, using "they" is becoming increasingly accepted as a neutral option.
Ultimately, whether "they" is considered vague or ambiguous depends on the context and the audience. It's important to be aware of the potential for ambiguity and choose the pronoun that best fits the situation.