In this investigator's humble opinion, the 'orb' is one of the most contentious and discrediting phenomenon in the paranormal field. Like 'ectoplasm' ie. usually cigarette smoke, breath on a cold day, dust or otherwise, orbs are incredibly easy to debunk, extremely difficult to support and can bring damage to the reputation of any investigative group in all but the most extraordinary of cases.
Thanks to modern media (tv, internet etc.) the orb has become one of the staples of the paranormal investigation, with wide publicity from shows such as 'Ghost Hunters', 'Ghost Adventures' and the like. However, when closely considered, they are generally little more than filler for lack of other evidence.
When reviewing photos/video of these phenomenon with a skeptical eye, they are easy to discredit, weaken or outright debunk due to the ambiguity and lack of support that they are generally accompanied by. Thus, use of them as 'evidence' is both a slippery slope toward wishful thinking and projection of intent.
In the vast majority of cases, orbs are dust, either free floating in the environment, or attached to a camera lens. The fact that we film/photograph in the dark, using a directed artificial light sources (either white or IR), is the main culprit in such events. Where in daytime filming, ambient and natural light are the source of illumination, in the dark, the light sources provided by the investigator's equipment are the only light source.
As most well know, our vision, and similarly, the light sensing capacity of our gear is dependent of light photons being reflected off of an object and that light (minus various EM frequencies absorbed resulting in color) has illuminated. In ambient light, photons are coming from various if not all directions. This produces the full illumination that we generally enjoy during the day, or in a lighted room. In such environments, it's unlikely to see an 'orb' as the light reflected toward our eyes and equipment drowns out the reflected light of all but a few small objects in the air, or on our lenses.
However, when filming in the dark, this is not the case. The light produced from the equipment is projected in the specific direction of the intent to capture, resulting in a direct reflection of those things in the atmosphere, without simultaneous light sources drowning it out. In this, the photons are directed straight back toward either the eye, or the camera creating a highly illuminated spot in a relative void of light. This gives the appearance of a free-floating ball of energy. This is exacerbated by the fact that the camera will not be able to adequately focus on this spot due to its small size, and will chose instead to focus on the larger aspects of the environment, thus creating an apparently larger, spherical shape which characterizes most of these phenomenon.
As well as dust, this can be caused by a variety of sources, from water droplets, insects, clothing fibers, pole and the like. Further, the fact that in many of these shows, they identify these things due to their apparent 'intelligent' movement, as proof of paranormal. This is also easily debunked with simple physics.
When we move, breath or otherwise disturb the air in the environment, we create currents, albeit sometimes very subtle that will affect the movement of anything suspended in the air. This can cause these free floating reflective particles to move in unexpected ways, such as toward, away from or around a person or object. This is also true of natural air currents in the location. Furthermore, in the case of small flying insects, it's obvious that they are self-propelled. As well, one consideration that is frequently neglected is static electricity. When moving through an environment, particularly a dry one, our clothes and movement will generate an electric field, which will also attract, repel or disturb small and easily influenced particles in the air. This is why frequently; the orb is seen to 'go into' a person. It's the dust being quickly drawn to their static field, causing it to accelerate toward the person.
Therefore, given the compelling and broad range of ways to debunk 'orbs', the consideration of using them as evidence of the paranormal serves more to open up an investigator or group to significant questioning of their ability, professionalism and integrity.