Here's all you want to know about telecine (TK):
http://en.wikipedia.org/wiki/Telecine
The short answer to your questions is:
Yes you can shoot it off a wall or use a cheap telecine box, but the resolution and possibly sharpness will be reduced, as well as the latitude coming down to video, and you will even encounter flicker, but yes, it can be done.
The advantage of a proper telecine is well worth the money. It is directly projected onto a calibrated CCD or other imager, and done with the 2:3 pulldown flag, so that you can perform an inverse telecine (IVTC) in your Non linear editor, and pull out a perfect 23.976fps, and there should be no flicker. Additionally, the controls at the TK itself are incredible for being able to flatten the latitude of the image so that you can then turn around and crush it / color correct and tweak to your desire in post. This is essential, because you don't want to lose the latitude and have a blown out image when you sit down to do your tweaking in your NLE. Finally, if you slightly underexpose or over a shot, the TK can pull a certain amount of it back, whereas if you copied it off a projection, it would basically be lost forever.
Really, it isn't so much about color correction, because if you are finishing the project digitally, all NLE's have very sophisticated CC controls, or if you are finishing on film, the color timing and grading comes at a later point. It relates to finishing digitally by allowing you the maximum resolution, latitude and perfect fps synch before you get into your NLE, rather than hampering your efforts before you even get started.
It should also be noted that for very serious projects on maybe higher formats than s8, some places offer uncompressed 4:4:4 TK, for the same price. Here in the states, Cinelab does, and I think 3516 does, but I'm sure there's more. It requires more hardware, but trust me, if you are wanting to do CC, NOTHING beats uncompressed.