"To reduce spambots" does not seem like a solid enough reason. Stack Overflow and Stack Overflow Chat have community-driven moderation. Also all Stack Overflow registrations already make members jump through the "Spambot" filter hoops (e.g., email confirmations / CAPTCHAs).
We wouldn't/don't restrict the posting of Questions on Stack Overflow to 20+ reputation members, so why limit Chat? It's the "new" members/programmers that really need the ability to "talk" to other members. Many aren't ready to formulate a Question, or knowledgable enough to commit themselves to an Answer, so getting that 20+ reputation takes time.
Considering the cost on new users, is the restriction really necessary?