Some are really beautiful, all of them are at least neat looking. Gives you something really different to talk about. They carry no diseases. Like fish they are enclosed in a tank, no mess to step into. Are great for breaking the ice with new people.
Tarantulas are a great conversational piece.
They teach discipline, not the easiest to care for so you must stay vigilant. Keeping one can change mild arachnophobes into arachnophiliacs. You'll learn latin fast. Some people are intimidated of them. This fear can range from mild to a extreme phobic of spiders.