Because it removes very bit of oxidation in the pattern and your silver will end up looking like chrome. Pieces selling on the web that have been dipped do not bring as much as those which have not been dipped and either been polished by hand.
Also tarnex will make some base metals black, so if you have an area of plated ware that is worn, it will make it appear worse.
I have a couple of very plain silver bracelets and I will use it for tham....and If the space between the tines on a fork are really black, I some times dip just the tips of the tines in tarnex rather than try to get ta the tarnish with a traditional polish.

I'll second Lindac's explanation.

Third...

Thank You.

thanks