First: freeride on a glaciers = high alpine risks
Not only crenvass, but also weather (in case of storm the temperature drops very fast), orientation (in case of fog and "white out" find your way is challenging without map/compass/GPS), far from rescue point and often without phone signal, ...
So if you are talking about freeriding routes far from the groomed slopes a complete mountaineering formation/training is essential. Of course this will involve also rope operations, the use of the ice axle, of the harness and so on...
This is a couple of example of freeriding in a challenging terrain requiring complete mountaineering skills:
http://www.skiweekend.com/sidecutandrocker/schwartztor-porta-nera/...
http://www.valleeblanche.it/?lang=en
If you are thinking and talking of freeriding near-the-slopes in the glacier resorts, wearing an harness (and carrying at least a rope) depends essentially of the characteristics of the glaciers and of the season.
Glaciers: there are glaciers with a lot of "monster" cracks and other with small, not so wide, cracks that could be well filled by the winter snow. Furthermore, if your route is in the middle portion of the glacier and you'll avoid the "end crenvass" at the bottom and the "terminal" crack at the top, you'll ride the less dangerous section of the glacier. As general rule, knowing your terrain is fundamental. The best is if you have seen the glacier in the summer, when the snow cover melts and the cracks are visible.
Season: During late winter and early spring often the amount of the snow (in "normal" years, not the case of the last one!) covers very well the crenvass and you don't need anything that your avalanche safety kit. During starting season (like now) the snow bridge over the cracks are very thin and it's very dangerous to ski off piste. Harness or not. The only way to freeriding safety is to know very well were the cracks are located (tour with a mountain guide?)
So in the end, I didn't answer you.
I'm telling to you, invest on a mountaineering course and then you could be able to evaluate when and where the harness/rope/axle/crampons are needed and in case how to use them...