A ti o No a ti
 

Solo te quiero a ti.
Solo te quiero.

Is the "a ti" needed or just for emphasis? Seems redundant to me.

In general "a ti" is used for emphasis.

However, the meaning of this particular sentence implies emphasis ("I love only you", or "The only one I love is you"), so perhaps in this context it sounds better with "a ti".

I wonder what our native speaker friends think?

I think "a ti" is necessary in this context, because it wouldn't be clear what you mean with "solo te quiero": Would it be "I love only you" (which would only be clear with "a ti")? Or "the only thing I do is love you" (which would be the first idea I'd get without "a ti")?
For me, it's needed more for reasons of clarity than for reasons of emphasis. :)