I would reckon a remake is only neccesarry if there can be a significant update with regards to visuals, sound and interface (e.g. ultima series), or if the original was a great game but bugged and flawed and just needs to be made more user-friendly (e.g. daggerfall).
Also, please check out some fantastic people with regards to sierra remakes...
http://www.agdinteractive.com
http://www.infamous-adventures.com