1 Thessalonians 5:19 Do not quench the Spirit; 20 do not despise prophetic utterances. 21 But examine everything carefully; hold fast to that which is good
1 Corinthians 14:29 Let two or three prophets speak, and let the others pass judgment.
At times I think that the chapter-and-verse reference system – not part of the original texts – that aids in referring to and finding specific Scriptures – can be distracting or misleading. It is visually tempting to perceive each verse in 1 Thessalonians 5:16-22 as a separate and distinct statement. While Paul was winding up his letter, giving brief instructions, they are interconnected, forming a unified context. Thus, the instruction not to quench the Spirit is linked to the instructions not to despise prophecies and to think about such prophecies and retain what is good. Taking these verses from 1 Thessalonians 5 together, four things are worth pointing out. First, Paul would not forbid despising prophecy unless that were an actual problem in the church at Thessalonica. 1 Thessalonians being in part a corrective letter – addressing problems in a church – it's a reasonable to infer that these verses were intended to be corrective. I think that very correction is relevant and needed today! Second, despising prophecy would have the effect of quenching the work of the Holy Spirit. Besides discouraging prophecies, it would also tend to hinder the expression of other Spiritual gifts and the work of the Holy Spirit in the congregation, generally. Third, utterances purporting to be prophecies were to be considered carefully and judged. These are the two errant extremes: reflexive rejection; credulous acceptance. Persons “prophesying” could be mistaken (or worse, of course); what was done in such a case, beyond rejecting the message, is not mentioned (a prophet judged to have been mistaken or wrong would not, of course, have been killed, as in the Old Testament). My guess is that the character and severity of the mistake/error and the character of the person would have been key considerations in what would be done. Fourth, a prophecy judged to be true prophecy was to be treated as very important (though not as Scripture, as I pointed out above).
The big picture – in 1 Corinthians 14 and 1 Thessalonians 5 – is that prophecy (and all the other Spiritual gifts, for that matter) was/is not a toy or a tool for self-aggrandizement. Prophecy was given – as a gift and as messages – for the benefit of assembled believers. Paul communicated this clearly in these two letters, and likely taught this wherever he established a church.