The fundamental question is, Is God's specific guidance and direction necessary and relevant for church today?. Absolutely. And given that, prophesy is necessary and relevant for today. For that is all prophesy is really. Does this mean the bible needs completing? No, the bible is finished and complete. And it's a shame to tamper with ancient historical documents anyway.
That doesn't mean we cant write a book that is inspired that addresses other things.
For example, look at all the christian books around today, dvd's etc.
Obviously, the bible alone is not enough for many Christians.
Excellent point.
Very good point, and exactly right. God has no more to say on these things.
What He does have to say though, is how He wants His church to best use the time they have now on earth, to fulfill His plans and purposes until Jesus returns. To fulfill the plan of salvation and bring it to completion.
And for that reason, we need people who hear the voice of God and obey it. Whether it be through the Word or other ways. While it is correct to say we hear Gods voice through the Word, we need to know which parts of the Scripture God wants us to read, and which parts are standing out to us. We cannot use our own mind to interpret what we think God is saying, but must even interpret the bible with the Spirit's help. Or if you like, we need people who can read the bible and know which parts to read and which parts God is speaking through, that are relevant for the situation today and tomorrow. That all comes under the general heading of "prophecy".
God did NOT, set up the church , close the canon, cease prophesy, and then expect His people to use their own human effort, intellect and what THEY THINK IS RIGHT in their own ways and mind, to fulfill His salvation plans for the next 2000 years. That is why many modern day churches are structural worldly organisations centred around pride, greed, and money, and control, rather than doing what they supposed to be doing.