It's by virtue of the fact that it's fully manual. Unlike most lenses made over the last decade, there's nothing digital or electronic in there, it's all moving parts that you can move yourself. So you will never not be able to focus with it or control the aperture. The downside is that you have to do those, and not the buttons on the camera, but to be honest, any photographer worth their salt does this anyway.

As for the second part, I'm not entirely sure what you'd be losing when mounting the Helios onto the M4/3rds. From a quick read through of the wiki page on that system, it would be appear that cropping occurs and a slight loss in image quality due to the sensor. I have underlined a few points of interest.

From the page:
Disadvantages of Micro Four Thirds compared with DSLRs
    The sensor is 35% smaller in area (2.0x crop factor) than APS-C (1.5x crop factor, or 1.6x for Canon-APS-C) sized sensors and 75% smaller (i.e. a quarter of the area) than a full frame sensor (1.0x crop factor) (35 mm equivalent), which can mean lower image quality when all other variables are the same. This might include poorer color transitions and more noise at equivalent ISO settings, especially in low light, when compared with the larger sensors.
    Due to the absence of a mirror and prism mechanism, there is no ability to use a through-the-lens optical viewfinder. A through-the-lens electronic viewfinder, an attachable optical viewfinder (similar to a rangefinder or TLR), or the universally supplied LCD screen can be used instead.
    A larger crop factor (2x multiplier versus APS-C's 1.5x) means greater depth-of-field for the same equivalent field of view and f/stop when compared with APS-C and especially full frame cameras. This can be a disadvantage when a photographer wants to blur a background, such as when shooting portraits.

Popularity with Adapted/Legacy Lenses
    Due to the short native flange distance, of the Micro Four Thirds System, the usage of adapted lenses from practically all formats has become widely popular.
    Because lenses can be used from old and abandoned camera systems, adapted lenses typically represent good value for the money.
    Adapters ranging from low to high quality are readily available for purchase online. Canon FD, Nikon F (G lenses require special adapters), MD/MC, Leica M, M42 Screw Mount, and C-mount Cine lenses to name a few are all easily adaptable to the Micro Four Thirds system with a glassless adapters resulting in no induced loss of light or sharpness.
    Due to the 2x crop factor of the Micro Four Thirds System however, most adapted glass from the 35mm film era and current DSLR lineups provide effective fields of view varying from normal to extreme telephoto. Wide angles are generally not practical for adapted use from both an image quality and value point of view.
    Some disadvantages of using older adapted lenses on micro four thirds sometimes slight losses in image quality. This is the result of placing high resolution demands on the center crop of decade old 35mm lenses. Therefore 100% crops from the lenses do not usually represent the same level of pixel-level sharpness as they would on their native formats.
    Another slight disadvantage of using adapted lenses can be size. By using a 35mm film lens, one would be using a lens that casts an image circle that is far larger than what is required by Micro Four Thirds Sensors.
    The main disadavantage of using adapted lenses however, is that focus is manual even with natively autofocus lenses. Full metering functionality is maintained however, as are some automated shooting modes (aperture priority).
    An advantage of wildlife shooters and birders in particular lies with the fact that old 35mm telephoto lenses become extreme telephotos due to the system's 2x crop factor.
    Overall, the ability to use adapted lenses gives Micro Four Thirds a great advantage in overall versatility and the practice has gained a somewhat cult following. Image samples can be found readily online, and in particular on the MU-43 adapted lenses forum.