Why get an expensive tuner?Am I missing something?

It depends on the radio stations you listen to. A good NPR or college station (read: non-compressed signal) can sound as good as, or better than CDP or turntable. So the answer to your question is yes a good tuner really makes a difference.
Cheers:~)

>as good as, or better than CDP or turntable

Waitaminute. How can it sound better than, unless perhaps your CDP or turntable are really poor?

I can say that until I got a Day Sequerra I was content to use the tuner for background music. Once I heard what a great tuner could do, I got a top notch outdoor antenna with a rotor and have been listening a lot more seriously since. A great tuner can certainly help you in finding sonically good CDs (even taking into account the dynamic compression on the big orchestral stuff, you can hear the sonic merits of a recording), and when you get the occasional live broadcast (on WQXR here in NY, you get the Met Opera broadcasts, NY Philharmonic and, best of all, small ensembles in QXR's studios) it is really something special and can outperform a recording (Drubin, that's where in my view it can outperform a CDP or TT; or, of course, if the station uses a Burmester or similar CDP and a Rockport TT, then maybe it can outperform a listener's system, but that's not really likely, and I do feel that the dynamic compression the stations use can limit the material that would sound better). Plus you get a lot of great music, of all types, for free!

While rare, there are some good quality broadcasts available. A good tuner can avail you to a multitude of quality music you might other wise never know about. While I would love to own a really top notch tuner for the above reason, there are some bargains out there. Don't completely cheap out here. If your tuner sounds bad, you won't use it.
Think of all you might miss. Antenas are as important as tuners, especially re: tweaking for best results. Unfortunately a little bit of luck as far as your location, goes a long way re: both available broadcasts and reception quality.

The first thing you need is a great signal. If you're lucky your favorite station uses utra high end source gear. At that point you might find that a tuner will beat your TT or CDP. I've listened to a Naim NAT01 that sounded like an upper end CDP.

-Patrick