@Reborn: reading the wiki will help you clear your thought.
A Space Marine does not need to be "stealthy" as we think it our present day. If you have to sneak into a palace and kill someone you send an assassin.
Scouts and snipers are new recruits and don't wear full power armor. You will use them to scout the battlefield or gater infos.
Space Marines has various implants, and the Omophagea allows them to... "gain part of an individual person's or creature's memory by eating its flesh. This implant thus allows a Space Marine to literally "learn by eating." It is a lot more efficent method than crouching around and use your eyes and ears to know things.
More, the Space Marine Power Armour is all but just a layer of metal to absord "few" ballistic or melee attacks.
- It enhances (not encoumber) the Marine movements, keep him fast and balanced.
- It protects the Marine from all the dangers coming from planet environment (not all planets where Space Marines fight are terra-like) and the vacuum of space (just in case).
- It helps to heal wounds and all other sorts of damage the Space Marine suffers, during combat or after for quick recovery.
Considering that melee scenarios are common in 40K as well as long range engagements, you know why having a super resistant armour, that makes you faster, stornger, more balanced and can absorb tons of damage while healing and protecting you is a huge asset.
And when you are in messy situation or you need to fire your weapons quick and efficently, is the Power Armour that hosts the Auto.System: a fully integrated tactical targeting and threat analysis system.
Considering all of this, calling a Space Marine in full power armour "clumsy" is the opposite of truth. And by the way, Space Marines knows perfectly how to move silently and conceal themselves in the battlefield if needed.
All of this is hard to imagine looking at the "canon" image of a Power Armour. If can help, imagine them like this, instead of the images you posted: