More often than not tho, the speed of the medium and light armor is totally useless. When you're pushing into a chokepoint in LV caves for instance, there's nowhere to move but forward. If anything the speed of medium/light will land you in hot water, as you'll advance faster than you can count the marines around you to coordinate. And when you inevitably get killed leading the push, you'd best believe you'll be sent up to medbay if you were revived wearing medium or light.
If the fighting is exclusively in tfort or perhaps even outdoors in big red/trijent, then you might actually get some use out of those speed bonuses. In general though, caves tend to be the hardest, most grueling part of a marine push where it really comes down to keeping marine numbers up thus out DPSing the xeno mob's regeneration.
You might get a few more peripheral kills with medium/light, but that (usually) doesn't result in a marine victory. The biggest determinant of a marine win in my experience is being able to grind the main front into the hive, and still having enough marines around to score kills on the weakened xenos driven to defend it.
The biggest marine weakness is logistics; for the average pfc, medium/light armour just aggravates that. If your role involves a lot of running around (eg, medic, perhaps engie) and less damage sponging, that's where light armour actually gains some credibility.