It's all about context. Except for solo piano, you'll almost never use either the full spectrum or the full dynamic range of the instrument. In rock/pop mixes, it's common practice to severely thin out and compress the piano. If you don't, it'll step on just about everything else in the mix. So while the TTS-1's piano isn't very convincing in solo, there are times when it'll serve just fine within the context of a mix.

I'm a piano player (not that that matters much) and Bit has hit the nail directly on the head here. Anything with an 88 note range needs reigning in for obvious reasons. Also to be considered here is the piano arrangement within the song which can make a huge difference in terms of the type of sound needed for best effect.

Bibs

My thoughts are similar.... for me, most of the piano's I have will work. Most are sampled..... Kontakt and Cakewalk's Steinberg collection for the most part.
 
On a solo piano and voice song, the piano sample needs to be anywhere from good to top quality. If there are other things in the mix, the piano really doesn't need to shine quite so much. And even on a vox/piano mix, a decent quality sample will work just fine. You don't always need a Carnegie Hall level piano sample.
 
Depending on the song and the singer and the desired effect you are going for, the piano and even be pretty cruddy and still have a slamming song out of it. Have you listened to some of the music on commercials on TV recently?

I use Yellow Tools Piano almost exclusively. It's a pretty simple full range piano - yet they default with it using an incredible velocity to filter modifier. The filter is set to about 300 hz no less. Play it in the lower to middle velocity range and it's really dark. You can hear the felts hitting the strings - and it's perfectly smooth sounding and great for jazz and adult contemp. If you set the filter up a little bit around 500 hz it sounds extremely smooth and full range and gets a nice bite when you hit upper middle to high velocities. I tried to mimic this in Kontakt with NI Akoustik piano and couldn't get close. It's very dynamic and I've yet to find a better piano sound. I've used Kurzweils for years as well and this just blows it away in my opinion.