Well... Mike Mearls has gone on record as saying they intended that Versatile let you use the weapon for whichever situation you want. One handed to benefit from one-handed stuff, and then two-handed to benefit from two-handed stuff.
Now, I checked my PHB and his name is on the cover. Apparently he co-wrote the damn book.
If -that- is not rules-as-intended, nothing is.
But regarding Rules As Written:
Other one-handed weapons are large enough that you can keep a good
grip on them with two hands and deal extra damage
by using them as two-handed weapons
In the D&D4 rules templating, using something as something else permits you to benefit from feats/abilities/powers involving that something else.
For example, Wizards of The Spiral using a sword as a wand get to benefit from all wand stuff with that sword.
Using a songblade as a bard implement means you can benefit with all implementy stuff with that songblade (like getting that enhancement bonus.
Using a power as a melee basic attack when permitted allows you to use it at the end of a charge.
So, yes, using a one-handed weapon as a two-handed weapon allows you to treat it as a two-handed weapon. That's what versatile does.
Otherwise, it would do nothing except give you +1 damage, rather than allow you to switch between one-handed and two-handed if you're going for a flexible build. For example: A barbarian can use a versatile weapon and a small shield and still use his two-handed weapon powers.
Besides, it's grokkable. To say a weapon that says 'you can use this as a two-handed weapon' can't benefit from it is counter-intuitive. It doesn't make sense, and players' instincts don't go in that direction. It might make sense from a pure rules standpoint (maybe) but it doesn't have versimilitude.
Personally, I'd rather not let bastard swords overshadow two weapons for the cost of a single feat
Superior weapons get to do that. A fullblade overshadows both the greatsword (1d12 damage) and the greataxe (+1 to attack, while still getting highcrit)
You spend a feat, you get a superior weapon. That's kinda the point of it. If the bastard-sword weren't better than a greatsword for a feat, there'd be no point taking the bastard-sword if you weren't a halfling/gnome.
And a bastard sword doesn't overshadow a greatsword with a feat... at higher tiers the greatsword with feat does more damage. If it's a goliath+greatsword+feat, the greatsword does more damage in heroic. Comparing a weapon that requires a feat to use with a weapon that doesn't, but without the benefit of that extra feat is NOT a reasonable comparison to make.