Steam Trade Offers clearly states it's only available to friends. But they may expand this later. Meaning that no you will not get offers from random strangers. You can also already limit who can see your inventory to friends or friends of friends. So there is no reason for this to be a problem unless you are stupid and don't check your options.
Regarding MS XB180, Steam does not require you to be connect at least once every 24 hrs. Steam has an offline mode so you do not need to be online for long periods of time. To my understanding, only asking for you to go online if you have a net connection and it detects there is an update for your game. MS wanted to require a connection every 24 hrs regardless what you were doing...even playing a retail disc, single player game.
I like to take my PS3 to friends or family homes that do not always have wireless setup or a hard line available. MS's attempt at required online was, as already said, entirely self serving and did not benefit the user in any way.
And if you buy into the "cloud" advantages spoon fed by marketing, you are exactly the idiots MS was trying to sucker.