A prominent investment adviser to the banking industry today warned against use of "the taxpayers' money to bail out incompetent management" of either thrift institutions or banks.

Harry Keefe Jr., chairman of Keith, Bruyette & Woods Inc., told a conference here on the future of the depressed thrift industry that mergers with stronger savings and loan associations or banks would be preferable to costly federal bail-outs. He also saw the possibility of converting some thrift institutions to stock-issuing corporations.

John J. Mingo of Golembe Associates Inc. of Washington, D.C., added that commercial banks may not only be among the few viable bidders for the larger troubled savings institutions, but would be in a better position under the new tax law to convert S&L's losses into favorable tax credits. He suggested that acquisition of thrifts would also be attractive to banks seeking to penetrate new markets.

Wednesday, Richard W. Kopcke of the Federal Reserve Bank of Boston told a conference being sponsored here by the bank on thrift industry problems that two out of every three savings institutions are already insolvent, which means that the nation faces a bail-out cost ranging from $30 billion to $200 billion.

Keefe said that "it won't do any good to give these basket cases an infusion of cash that just helps them to continue what they are doing today." Keefe said that "there are too many banks and too many thrifts," and that management of the thrifts had paid excessive attention to subsidizing mortgage-holders at a serious cost to their depositors. He said that thrift managers instead should be seeking interest-sensitive investments that would protect their depositors.

To critics in the audience today who argued that thrifts were established to support the housing industry, Keefe responded that if it is national policy to subsidize homeowners, the government itself, rather than depositors in the thrifts should put up the subsidy.

But a number of other speakers at the conference today argued that despite the enormous problems of the thrift industry, many S&Ls have a bright future, and these should have available to them some short-term assistance.

Those S&Ls that do not have any hope for long-term profitability, according to University of Houston professors Paul M. Horvitz and R. Richardson Pettit, "should be merged out of existence as soon as possible."

Horvitz and Pettit estimated that savings and loan institutions lost $1.5 billion in the first six months of this year, will lose an additional $3 billion in the second half--and yet another $4.5 billion in 1982. But many institutions, they said, have surplus accounts ample enough "to afford these losses for some time."

The institutions that are judged to be potentially profitable should be allowed some relief both from government regulations as well as some form of cash aid, the Houston professors suggested, primarily because in their view the problems of the industry have largely been caused by government.

"The reason that thrift institutions can continue to operate even though the value of their assets is less than liabilities, is that the bulk of their creditors are insured depositors who have confidence in the federal deposit insurance system."

They noted that in the past, when the question has been raised whether or not the assets of insurance funds backing the banks and the thrifts might be insufficient to cover failures, the standard answer has been that such an event is impossible.

"For the first time since the creation of the FDIC Federal Deposit Insurance Corp. and FSLIC Federal Savings and Loan Insurance Corp. , that question has become a relevant one, and therefore the traditional answer has become less convincing," they said.