I would like to convert Pantone to CMYK?
What are the dangers of doing this?
All printers I know use the Pantone system. Colors on press can always differ from run to run whether it is a Pantone or a process color. The only way to really guarantee that you get the color you want is to bring your own swatch book and attend a press check.

Printing is just as much art as science and colors can differ based on different brands of paper and their finish, the type of press running the job, the skill of the pressman, the lighting used to evaluate the color and even the humidity in the air.

Many printers have these expensive viewing light boxes and I think it is ridiculous. I always take the press sheet and bring it out into the office area and/or take it outside need to assimilate the same conditions under which my clients will be viewing the piece.

Logically, when you get a file in Pantone specs and want to change them
to CMYK you convert them using your software. Right? Well it does not work that way. The colors don't turn out right. Can anybody tell me why?
Pantone Matching System (pms) colors are made up of variations of cymk - I believe you are referring to what is called a spot color - spot colors could be a pms color that is made of a solid form of ink and on the computer in different software is titled different things. Each software operates differently and colors looks different at times- this is when you are supposed to be designing with knowledge - a cymk breakdown of a pms is the exact same in every software even if it looks different on your screen - due to software - different operating systems - monitors - etc...

My biggest problem has been clients who don't have their own logos in hard
copy or on disk. Or even available for download on the net. They give me business cards printed in light blue on blue stock and say "here's my logo." One client gave me a beautiful letterhead, yes, in blue. I can only do so much magic.
My solution has been to provide a "Slick Sheet" of client's logos ONLINE as Illustrator EPS files. Any time the client needs a logo sent to a vendor (for whatever, newspaper, T-shirts, whatever) all they have to do is refer the vendor to a secret web page for FTP.

Trying to get some large companies to make logos available electronically via the Internet is harder than pulling teeth or amputating your limb. I am a strong advocate of this, as we charge every time we have to amyl or send a logo on CD (no floppies anymore!), and this cost is usually $35/instance