The conventional wisdom on the new All Savers certificates has it that this tax-free investment is useful only to a person in a federal income tax bracket of 30 percent or higher.

To be honest, I have implied much the same thing myself in my financial advice columns. But further reflection has convinced me that this is not necessarily the case, and that in fact, the lower income investment may turn out to be the principal beneficiary.

What is perhaps more disturbing is that if I'm right, the whole idea may mean greater trouble for the thrift institutions instead of being the savior that was expected and hoped for.

The All Savers certificates are available at banks and S&Ls during the period Oct. 1, 1981 through Dec. 31, 1982. The certificate is issued to pay interest at 70 percent of the rate on the last previous sale of one-year Treasury bills.

Thus the interest rate will be different for certificates bought at different times; but once issued, the rate on each individual certificate is fixed for its one-year term.

Interest earned on these certificates is exempt from federal income tax and--depending on where you live--may be exempt from state and local income tax as well. The maximum "lifetime" exclusion is $1,000 per taxpayer ($2,000 on a joint return).

Here's how the numbers work out. If you're in the 32 percent tax bracket and buy a one-year $10,000 T-bill paying, say, 16 percent, you get a return of $1,600 and pay federal income tax of $512 (no state tax liability).

That leaves you with a net cash return of $1,088. But if you buy a $10,000 All Savers certificate you will earn $1,120 (70 percent of $1,600) with no tax liability--obviously a better deal. And of course it gets even better if you're in a higher tax bracket.

On the other hand, for a taxpayer in the 28 percent bracket, the net after-tax return on the T-bill turns out to be $1,152--$32 more than you would get on the tax-free certificate. It's this kind of arithmetic that leads to the idea that the new certificate pays off only for the 32 percent-plus type.

This is a very logical and reasonable theory--on paper. But in the real world things have a way of working out a little differently than they do on paper, or even in a super-duper computer model.

So let's look at that real world. A family of four in the 32 percent tax bracket for 1981 has an adjusted gross income of at least $32,000--more if they itemize deductions. And of course a higher tax bracket means a still greater income.

Most people with that kind of income can be assumed to know a little about investments--enough to be at least on speaking terms with such things as tax-exempt securities and money market funds.

Some of these people may move funds from other investment media to the new certificates. But I think many are apt to take their time and consider such a move pretty carefully.

And more often than not they will realize that in their particular circumstances All Savers certificates may just not be the best thing coming down the pike.

But if that judgment is true, who were all the people waiting in line to buy certificates at the beginning of the month? And where did all the money come from that went into those early purchases?

I believe the principal appeal of the new certificate is to the millions of totally unsophisticated folks (unsophisticated only in terms of investing "smarts") who like the security of FDIC and FSLIC insurance.

Municipal bonds, stocks, "repos," most mutual funds and the various tax shelters are not protected by federal deposit insurance. But the All Savers certificates are--and aside from the tax exemption this may be their most appealing feature.

Here are tens of millions of people whose main concern is top security for their savings, with those savings sitting in bank or S&L accounts earning less than 6 percent.

Suddenly along comes an opportunity to get more than 12 percent on the same money in the same bank or S&L with the same federal insurance.

To these people the tax-free feature has to be secondary--just icing on the cake. You don't even have to understand what "tax-free" means to know that 12 percent is better than 5 1/2 percent. So even if you're in the 21 or 24 percent tax bracket--or in fact have no tax liability at all--movement of the maximum amount from a passbook savings account to an All Savers certificate makes economic sense.

(You just have to be sure to use only funds that you don't expect to need for the next 12 months--there's a substantial penalty for early withdrawal.)

I strongly suspect that's where much, though of course not all, of the money going into the new certificates is coming from.

Well, what's wrong with that? Here's a bunch of people who have been on the short end of the stick for years, settling for a return that didn't even keep up with inflation, now getting an opportunity for a little better break.

What's wrong is that the All Savers certificate was designed primarily to help the nation's S&Ls out of some deep trouble. They have a lot of money loaned out on long-term mortgages at low interest rates--money they have to pay much higher rates for in today's market.

The new certificates were intended to provide new money to the S&Ls at a lower-than-market cost by having the government (read "all of us") absorb the tax bite. It was hoped that this device would siphon money to the S&Ls from the money market funds and other investment media.

But if the bulk of the money isn't new money at all, but comes from their own passbook accounts--on which they had been paying only 5 1/2 percent--then the All Savers certificate at 12 percent is not an answer but rather a compounding of the problem.