I think a thank you note to interviewers is necessary because:
1. It shows gratitude, interest and courtesy. It also shows good breeding.
2. It's a way for them to remember you/keep you in mind. Appropriately worded, it would also give you the means to keep your foot in the door - tipo bang, "I hope to hear feedback from you regarding my application soon. I shall keep in touch to follow up on the status of my application..."
3. It gives you the opportunity to reiterate some points that were/were not covered in the application. For example, kung meron silang mga questions na hindi mo masyadong nasagot, pwede mong pahapyaw sagutin sa letter. (Mind you in the past hindi letters ang pinapadala ko, kungdi card).
4. It shows that you are thorough and you have attention to detail, lalo na kung natandaan mo lahat ng names ng tao na nag interview sa iyo. Kung maari, dapat may separate cards ka for every person who interviewed you, including the HR officer. At dapat speedmail/courier ang delivery ng card/letter mo para mabilis ang pag dating sa kanila...Kasi kung ordinary mail, baka mahalo sa mga sandamakmak na sulat na natatanggap ng mga HR/interviewer everyday. It should be personally/handwritten and should be longer than just "Thank You", but wag naman novela ang haba.
As someone who's done this before and who now is in a position to receive letters like this, it is very much appreciated and it does help the applicant (not necessarily secure the job directly, but gets him in a position where the interviewers/HR think/remember him/her).