While the disciples were given power to cast out demons (Mk. 3:12f), this is not the point or issue of Matthew 18:18. In this passage contextually, the binding and loosing had to do with the corporate decisions made in relation to church discipline or matters where the church is faced with a rebellious believer who refuses to repent of sin as clearly defined in the Bible. Even Matthew 16:19 has nothing to do with casting out Satan, but deals with the proclamation of the gospel message (believe and be saved or reject and remain lost) which Peter had the priviledge of presenting first after Pentecost.
Further, the great emphasis in the epistles is never on attacking Satan, but preaching the Word and building men and women in Christ which enables people to draw near to God and resist the devil. While Satan does promote evil of every sort, the “binding and loosing” approach becomes an excuse for people to ignore the real problem—personal sinfulness. Thus Satan gets blamed as the immediate cause when in fact he is more a secondary cause. The more immediate cause and issue is generally a persons own ignorance, lack of maturity, and failure to walk by the Spirit in intimate fellowship with the Savior.