It would be nice letting new users Comment instead of posting an answer requesting for more info About the question/Issue
Flag those answers as not an answer, and it will be taken care of.
Spam comments are harder to spot and moderate. Answers that are added by new users will be listed in the review queues (any user with 500+ rep will be able to review them), and will bump the question to the top on the homepage.
What if someone posted spam comments on older questions of inactive users? Unless mods have a tool for that, it is not possible to spot new comments except if you spend your time on http://data.stackexchange.com/ubuntu/query querying for new comments.
The rate of spam that will be available due to comments being allowed by new users, is totally not worth it. Also think about the
Me too, etc. comments that new users will start adding; Who most likely won't know that they are not allowed.