Its true that rich people often (always?) "have debts", but not true that they are "in debt" overall, otherwise they wouldn't really be rich anymore. Poor and middle-class people are often in debt in the sense that they have borrowed against their future earnings. For things like a house, borrowed over 30 years, the inflation on that amount really does benefit them. By the end of the loan, people who stayed in one house barely notice the cost of the mortgage, and if the intrest rate is low enough they may choose to deliberately not pay it off as they invest their current cash elsewhere. For things like a credit card, no the inflation doesn't really help anyhing.

If deflation were expected, rich people really would just leave money in a bank account (as long as investing gave smaller returns, or seemed too risky.) The inflation is an incentive for cash-rich people to put that cash to use instead of sitting on it. This can be a huge driver for the economy.