There's currently not much reason to keep around a long-term non-deductible Traditional IRA in my opinion -- a Roth IRA is almost strictly better. Think about it: a non-deductible Traditional IRA vs. a Roth IRA of the same amount. In both cases, contributions are after-tax (so no tax deduction). But when you withdraw, for the Roth IRA you don't have to pay tax, and for the non-deductible Traditional IRA, you have to pay tax on the "earnings".
A Roth IRA can be contributed to at pretty much any income level, thanks to the backdoor Roth IRA process (which uses a temporary non-deductible Traditional IRA in the process). So there is not much reason for a long-term non-deductible Traditional IRA.
As for your question, a non-deductible Traditional IRA vs. a taxable account. Well, a non-deductible Traditional IRA is contributed to with after-tax money, and taxed on the earnings only on withdrawal. So the taxation is almost identical to things like stocks and homes, where the gain is not realized until the thing is sold. However, compared to things like savings accounts and bonds, where you get taxed on the interest yearly, it is much better. Every time you get taxed on gains like this, it is taxing gains earned from after-tax money, so if you think of an amount of money as being equivalent to the amount of money it grows to over time (time value of money), then it is taxing money that is (or grown from money that is) already taxed. So it is better to have this only happen at the end at withdrawal than every year.