Buybacks tend to help current top management when it receives compensation from stock options.  By "returning" capital via buybacks the stock price increases, when doing so via dividends, the price declines.

Until the Bush administration dividends were taxed at ordinary rates while capital gains were taxed at a lower rate, so there was some reason to do buybacks.  However, as you note, almost always investors would prefer cash in hand via a dividend.

I generally view buybacks as fairly self serving.

Preferred stock is basically just an unsecured loan.