You are right Dr. Bogusz, that many a times its the S & T/ Conference Committee which decides, depending upon sub-themes, time constraings, number of participants to be called for oral presentation etc. factors, which paper is to be presented as oral presentation and which one through poster presentation etc.
In my view, whatever be the reason (be its theoritical study or be it weak english or be it touristic participation as you've said or any other reasons), one thing is possibly sure that even in poster, what is generally reflected is the outcome of one study or another. It may be possible that the author her/himself is not ascertaining the quality/standard of the work concerned i.e. whether it is worth for oral presentation/ publication. But at the same time, S/he also wants to extend a form of publication to that particular work. Further, to be on positive side, it is her/his wish to exchange her/his experience of that particular piece of work and to impart what S/he has learnt and of course to learn some more inputs to what S/he has already done.
I think it is not for the researcher to decide, but the responsibility of the organizers of the conference to schedule which presentation ought to oral and which should a poster presentation. Again, the line of research may also be a factor. If your research is not in direct sync with the theme of the conference, but an interesting one and it is accepted, you may be scheduled for a poster presentation.
+ Some authors may prefer publishing the study in a peer reviewed journal as well by appropriately acknowledging the oral presentation. For some institutions the two may count as two separate publication entities. This is not the case when publishing the full paper on a conference.
I think Çınar Kaya may be right. Poster presentation (without a conference paper) gives an opportunity do discuss the results with the audience. And the contribution may be later published in peer reviewed journal without risk of self plagiarism.
In additions to the reasons named above, I would like to mention that conferences frequently encourage poster submissions for rejected papers to give the authors a reason to attend the conference (many universities only provide funding if "something" gets accepted). The organization of the poster session differs widely. In some conferences the posters are shown in a remote room with no one attending, while at others there are lively discussions. It fully depends on the organizers.
The technical committee of the conference organizer's seem to have tactically rated the oral presentations higher than those accepted as posters that's the reason many persons whose research works are accepted as poster presentations don't even show up for those conferences.
It is more specific in terms of attracting those interested in the topic, accessibility to the presenter is easier, it engenders more in-depth discussion with interested persons and it doesn't waste the time of persons who are not interested in the topic
In most cases, it may not be the choice of the authors but the editors or publication committee! Please understand that between oral presentations and posters, the former has more prestige. Whatever be the case, oral presentations and posters will not qualify as valid publications. For that you have to prepare and publish full length research papers or short communications in peer reviewed journals.
Mabrouk, I have used posters to present co-authored work partly because this serves as a good visual representation of what could have been presented through word. Additionally, a good poster always catches the eye and when done properly, the main research aim, research question and objectives (captured with mainly visual representations) could linger into the memories for sometime. Interesting.