That way lies Vancian casting. Been there, done that, don't wanna go back.
The extended casting times are features, not bugs, IMO. Sure, the memory-blasting ritual would be incredibly useful for getting past guards and the like. So useful, in fact, that it would severely devalue all the non-magical ways to get past guards. Hence the restriction.
Maybe YOU don't want to go back...
As for the utility of it...uhm...you get to use it once, and it costs you 7,000 gold. I'd call a delayed casting of it more of an "In case of emergency" than a "Rogue, stay home, we don't need you".
But think about the other circumstances you'd want to use it in. Sure, there will be cases when someone will willingly submit to it ("You must forget the location of the safehouse before you enter the lair of the mindflayer!"), but most of the time, you'll want to use it on an unwilling subject, and while you can engineer things to make it possible, it often reaches the point where it's easier to just kill them. ("OK, so we kidnap the prime minister, tie him up for ten minutes while we perform the ritual, and then make him forget he saw us stealing the silverware? And, oh, also, that we kidnapped him?")
Of course, if you blow the Arcana roll, you might just kill him anyway...
Which I suppose is a good reason to NOT let it be used without preparation, as it becomes a devastatingly effective combat spell. ("I make Orcus forget...uh...his mother's name. Ooopsie, rolled a 1! Guess he takes damage equal to his hit points now!")
I dunno, I am thinking a Utility power of level 9 or so which causes someone to forget the last, say, round, might be useful as a Daily power. Leave this ritual for the Big Stuff.