I don't know a whole lot about shielding technology, but that may have just been a cinematic tool they wished to convey along with the desert cloak. Alternatively, having sand bounce off of the shields could have been equally cool.

: The armor (as stated by others) is clearly battered and cracked. It's
: (supposingly) the same armor from Halo2 (but with upgrades). This means
: it's been through a LOT. don't forget he did crash land back on earth via
: a scrap of metal. The armor may be in dire need of repairs. The cloth
: seems to cling to his body more then if there was armor. Maybe he had to
: ditch a lot of it for his journey.

I figure that the techies aboard Infinity repaired and reinforced what they could with the antiquated Mark VI suit when he took it off (Halo 4 post-credits). His armor looks like it has been beat up since Cortana bit the digital dust.

: Sand is sand. If causes a LOT of damage to equipment. Regardless of how well
: it's sealed tight, sand WILL find a way in. Same goes for water. However,
: where water will eventually evaporate, sand will not. Instead, sand clumps
: up. It often creates a new seal/plug that spreads apart components as it
: does this. Taking in what we know about MC's armor, it needs to breath to
: allow change in internal temperature. It also doesn't have a limited
: supply of oxygen. When possible, the oxygen support system allows fresh
: oxygen in for the wearer. Now with a clearly very large dessert to walk,
: the flow of oxygen air and the adjustment of internal temperatures are
: vital to the person's survival. If sand builds up around those vents/fans,
: then there's BIG problem. Especially without a A.I to monitor it for you.

: The clock is clearly to protect the armor's venting system. And thus to
: protect the Master Chief.
: It's also a advertising gimmick.

The cloak serves a number of functions. Protection against the elements, mainly, but also to camoflage him against his surroundings. Much like the Marine snipers above with their cloak-like equipment, blending into a hostile environment is paramount to survival.

John looks like he's been battered to death from a new conflict, his armor and situation reflect that perfectly. A looming 7ft tall man in a dark green suit of power armor is going to raise questions from any onlookers. Not only can a cloak mimicking the texture and composition of a sand dunes allow him to elude detection from close visual range and drones, but if he so decides to venture along the outskirts of a settlement, a person is just going to think that John is a large man - not a Spartan, not a lumbering enemy threat.

And yeah, advertisement gimmick. A cool one that people are obviously overanalyzing. Wait, what? Ignore any of these statements from me above. :P