Hi Dave43!
What sort of products are you looking at? There's so many choices out there, it's hard to pick one. As far as your question goes:
The protective sleeve is typically designed for transportation, not for use. Most sleeves, are made of a neoprene-like material that if it covers the fan outlet will absolutely cause cooling issues. If you've got your machine situated on a desk or table, there really should be no reason to have it in the protective sleeve.
Now if you're referring to the plastic enclosures, they're designed to allow line of sight access to all the important peripheral ports. I personally haven't seen a neoprene based enclosure for the MacBook but at the same time I don't quite see the purpose.
As for the travel bags, it all depends on the manufacturer. First off, you'll want to get a bag that's designed for a portable. Secondly, you'll want to make sure that the bag is capable of supporting your 13" MacBook without a lot of wiggle room. Sometimes the "one size fits all" bags aren't the best for smaller laptops, especially if they support 12-17" portables. As for protection, you'll want to make sure you've got all four angles covered for maximum protection. My bag has insulated cushions on both sides and thick, bottom flooring as well as a fairly thick flap that comes overtop when I close the bag. I called it the "vault" and it's proven to take a beating at airports, public transportation and back seats. Never once had an issue...
