Hi
First off I sympathize with your situation. I have been at companies in the past where senior management does not get the value of HR and shuts them down. Here's how I'd handle it.
The first thing I'd do is ask your direct manager or senior staff what are their reasonings for you not to be involved in these items? Did they have a bad experience with HR's involvement in the past?
The second thing I'd do is bring up a few points as to why HR needs to be involved form a legal perspective:
1. HR can counsel all hiring managers on the things that should be and should not be said during an interview
2. HR is/should be responsible for conducting any backgrounds checks/drug screens/reference checks. This ensures they are getting conducted properly, legally and consistently.
3. HR should also be involved in employee evaluations to ensure that all evaluations are conducted fairly and that issues that are brought up by employees regarding any issues of discrimination, un fair treatment and dissatisfication are handled propmptly and properly.
Without HR's critical involvment in these tasks, the company is opening up itself for countless opportunities for lawsuits, employee resiginations and other moral problems. You can scare them by saying that the average sexual harassment and/or discrimination suit award is nearing over $250,000+. It's been as high as $7million. HR is the neutral party invovled in ensuring that both the company and its' employees approach each employee related matter fairly and within the law.
Lastly, if you get no where in your efforts to get yourself involved in day to day activties, polish up your resume and get the heck out of there. I highly advise you not staying at a company that does not value and utilize the HR function.
Best of Luck,
Leslie