Twilight Sparkle has always been special.

If it weren't for her heroic desire to do something about the return of Nightmare Moon, the mane six would have never come together. In general, she has always had a disporportionate role in moving the stories forwards.

In order to prevent her from becoming a Wesley Crusher-esque "Canon Sue", the writers have worked hard at giving her personality flaws and rough edges that make her an entertaining and sympathetic character despite her special role in the Mane Six. Over time, highlighting these issues is going to make this role increasingly problematic.

Making her become an alicorn Princess gives her room to grow, a role in the world of Equestria that reflects the role she already has among her friends. It allows her to have her foibles while making it obvious that she is special.

Princess Cadence (yes, I know her name is "Cadance", but that spelling looks stupid) may be a bland character, but she's a nice kind of bland, a reassurance that alicorns need not be unapproachable goddesses. I suspect Princess Sparkle will be much more like her than Celestia or Luna.

I'll miss plain old unicorn Twilight, but I don't think that becoming a Princess will cause her to leave her friends behind. If anything, it will reinforce why her friendship with the rest of the Mane Six is so important.

Now, let's see what kind of curveballs the writers are going to throw. I'm very much looking forward to this...