I do think that interpersonal skills are a tremendous asset in the workplace, I do not think that people can just fake their way through without a good basis of technical understanding. Going so far to say they you only need 15% of technical expertise to succeed is a bit preposterous. This will also vary from department to department though and across job function. The interpersonal skills come in handy when needing to obtain approvals or input from other people and expressing your ideas. You will not be able to climb the organizational ladder and gain favor if you are not friendly, respectful, and communicative to your coworkers and customers. However, you can only go so far on that and will not be able to bring anything substantial to the table eventually if you do not understanding the basic principles behind your product. All in all, I would say its close to the 70/30 split suggested by Daniel Goleman, if not closer to 60/40.