A reply to an opposition is not absolutely necessary. However, the reply is often the first document which the judge (or research attorney working for the judge) will look at to determine what issues are still in dispute.
Your reply, if you choose to file one, should NOT be a reiteration of what your motion states. Rather, the reply should be a response to the opposition. As such, the reply should normally consist of a memorandum of points and authorities, and might also include a reply declaration.
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney.
Depends on the arguments and the pleadings. This is a legal strategy decision and if they're response was horrible I wouldn't waste my time responding.
This is just my opinion and not a comprehensive answer. You assume the risk because this answer may not apply to your situation depending on the facts.