I think you should call other dentists office and ask them their opinion? or check some medical websites out- like this one, which states just the opposite of what your dentist told you (scroll to bottom of page):
http://www.braceplace.com/faq.html
Sucking on fingers and pacifiers are habits that may cause teeth malalignment and/or developmental problems of the jaws. This happens to some degree depending on force and duration of the sucking habit. Finger sucking usually causes more of a problem than the pacifier due to the distribution of forces. The fact is that digits are sucked for a longer period of time. Therefore it is recommended that pacifiers be used if one of the other is deemed necessary by the parent.
Many times merely stopping the habit will spontaneously return the teeth and jaws to their proper position. It will certainly stop any additional movement. It still may be necessary to move the teeth orthodontically, even though the habit has been stopped. Therefore, it is very beneficial for you as the parent to attempt to reward behavior modification training to stop the habit. If this is unsuccessful or teeth remain out of alignment after 6 months, I suggest you see an orthodontist for his opinion regarding possible orthodontic intervention.
I hope this helps?