First of all, VOICE ACTING IS NOT A HUGE SPACE EXPENSE!
That's easily provable. Base Fallout 3, the single most talkative character in the whole game, who has to comment on nearly every quest with multiple outcomes, as well as having in-person commentary takes up a massive 0.3% of a DVD-9. In contrast, Oblivion loses more space than this to DirectX. 30 MB is approximately equal to... half of the Oblivion intro. Now, let's keep in mind that this is the single biggest talker in a game. A central character, if you will, and one who has to say everything at least three times, thereby making it probable that no TES character has anywhere near as much to say.
It's mathematically provable. 30 MB is not big. Especially when we've been shipping on single layer discs to this point.
Space isn't a problem now, and is unlikely to ever be a problem. ESPECIALLY if they continue will full VA. If Beth ever decides to grant the wish of those of us who want more than a half a dozen npc's to have more than half a dozen conversation points, who knows, space might become a problem if all of it has to be VA...
Second, voice acting provides the developers with a significant advantage in crafting a character. It's more precise than text can ever be. Morrowind, for example , has little to no indictation when a character is amused, angry, or just being a dike. You can try to guess it from the words, but it provides no actual textual references to indicate that the characters aren't talking in Ben Stein monotone. Compare that to Martin Septim. We can disagree on how we interpret his voice and facial cues. That happens daily in real life. However, we're pretty sure he said the same words with the same cadence and accent in your head as he did in mine. We just disagree whether he sounds bored or haunted, scared or in shock, etc.
What advantage does it provide in character crafting? I think the word you're looking for is characterisation (and even then, it's hit-and-miss). Full voice acting only makes every step of character crafting harder, longer and more expensive from a dev POV, which is exactly why the end result of that process in TESIV and Fallout 3 was so unsatisfactory for so many people.
Martin Septim might have been one of the more fleshed out characters, yes, but as per my previous post (and yours, funnily enough), one man's treasure is another's trash, and i thought that Sean Bean phoned the whole thing in. It's actually a pretty good example of why some of us would prefer to let the imagination suggest the persona instead. Besides that, Martin is a main-plot-centric npc in a role playing series which was built on "go anywhere, do anything". Everyone who plays the game once is going to run into him, but few people who create multiple characters for various career paths (you know, like what the series was originally intended for..) will follow the main plot. If the minor npcs encountered in alternate careers all have to be nerfed because some people demand full VA on a limited budget, then Beth has lost the plot as to what their own flagship series is supposed to be about.
Third, when I talk to people, I expect them to talk back. Of course, I don't expect an NPC to offer me directions on how to go to Bravil, negotiate my way to a particular house, find my way to a chest in the basemant, and then follow the road to an abandoned fort, climb to the top, find a specific arrow, and follow it to somewhere else, etc, etc, all spoken. I expect them to either give me instructions in phases, or just write it all down. But I also expect when I go "hey, where's the local Fighter's Guild?", they'll actually answer me directly, rather than hand me a note that says "it's south of the west gate, next door to the mage's guild". If that's too much to ask, whether for "role-playing" or "space" reasons, then I guess I have no interest in trying to agree with you whatsoever.
It could be said that nobody on this forum is actually talking to anyone, yet the overwhelming majority of people understand the tone and context of everything that is written, even if they happen to disagree with it. Right now you're reading this, and even if you choose to deny it, you're applying subtle overtures to your "reading voice" so as to suggest my own voice and persona in your own imagination. (Go on, you can apply "smarta$$" to this particular voice. I know you wanna).
If, on the other hand, you meant that last statement figuratively, as in you want to be able to ask various npcs about various mundane topics and recieve non-generic, varied responses, then yes, i totally agree. It's just too bad that it'll never happen as long as full VA is the vehicle of choice in TES.