I can understand that, but I'm not sure what they could have realistically done. More dialogue I suppose, but her dialogue can't go on forever. Eventually, it would have to come down to a selection of post-game interactions that could be repeated with her and I can't think of what those might be that would satisfy anyone's definition of meaningful companionship without them having to use their own imagination.
You can already travel and fight with her. You can already live with her in either the castle or the fortress (depending on your alignment). If you want her to live with you in your choice of Skyrim home, well, you can simulate that on your own provided you remain in the house with her and, if you must step out, then just don't leave her in the house waiting for you for too long.
What's left? The ability to bed her, I suppose. But unless Bethesda was willing to accept an ESRB rating of "advlts Only" (not a chance), then it's unlikely that this would have amounted to little more than a bit of suggestive dialogue beside the bed followed by a brief fade to black. Hardly thrilling and a player could simulate that him/herself right now by just imagining the dialogue and then closing their eyes for a few seconds.
I really feel she's perfect the way she is now in that she's almost completely silent. I can therefore stand directly in front of her and imagine any sort of conversation taking place between us that I can think of. I can't do that with a Skyrim spouse because they bombard me with repetitive dialogue whenever my character is within a few feet of them. Besides being annoying, it's also immersion-breaking.
Although, I suppose the argument could be made that this is, in fact, more realistic. I just now walked over to my real-life wife and stood in front of her saying nothing for a few seconds. She told me "What the hell's your problem?" I guess that's close enough to "What do you need?"
I agree with you. To be honest, when I heard Skyrim was going to include marriage, I didn't really know what to think, but as it was in Fable, marriage doesn't really mean a whole lot (even with the ability to have intercourse, it is just a silly diversion that doesn't add anything to the game). However, I think if they ever decided to add onto the marriage component in DLC, then marrying Serana would be something EVERYONE would want. By that, I mean if they add children into the game or spouse/child related radiant quests (such as the rescue quests in Dawnguard). In Fable II, the introduction of children into the marriage aspect of the game really made me care more about the fact that you could get married, or at least it made it some what more rewarding and interesting. I will never forget doing the radiant quest where you rescue your child in Fable II, it was one of my favorites just because it really made all of the heroics I was doing feel personal. Then when you find out that the main villian (his name eludes me for some reason) killed your family to prevent the continuation of the heroic bloodline... well that was the biggest driving force behind my hatred of him, adding onto the fact that he already killed your sister.
As it stand, I still want to be able to marry Serana, just because it seems like such a natural conclusion to the way all of the dialogue flows during the MQ in Dawnguard (and not to mention the fact that you can bring it up with her directly, rub it in Bethesda, rub it in). Whether you convince her to be cured and have a redeemed life with her or turn her into your vampire queen, the option just seems to make sense. I agree that it really wouldn't add anything more to the development of her character, and might sour it for some. However, with the way radiant quests work, it just seems to me that marriage could be a much more interesting addition to the game outside of having another place to sell goods and get money. Add the option to have your husband/wife give you errands or ask favors of you. Put in quests where they get captured by vampires or the Dawnguard or some other group. Add in children into the game and give them the same mechanics.
No matter what is done, the issue of repetiveness will always rear its ugly head, however, as you said, you can RP whatever you want to regardless of what is going on. Adding in radiant quests to marriage in Skyrim will just provide more justification for its presence outside of the meager reasons it exists for right now. Either way, I still want to be able to marry Serana, but I think if they improve marriage in the future with radiant quests, it will make more sense to be able to do so. The whole reason I would rate Dawnguard as a success was because it made NPC interaction actually feel more meaningful. Adding into the marriage component would add a whole lot more meaning to spouse interaction in my humble opinion, and I hope it is something they look into doing, because adding that functionality in would be easy to achieve.