网友应该主语句首大写和句末用句号。

One argument that you tend to hear a lot from the pro-tipping crowd seems logical enough.

句子主干为：One argument seems logical enough.

that从句为同位语从句，而不是定语从句。同位语从句为名词性从句，that为连词，在从句中不作成分。引导定语从句的that为关系代词（偶尔为关系副词），必须在从句中作成分。本句中，that在从句中不作成分：you 主语 tend 谓语 to hear a lot from the pro-tipping crowd 宾语。主谓宾齐全，不缺成分。that没有作成分。同时that从句的内容就是argument，从句与argument为同位关系。