Some fonts have proportionally spaced numerals, while others have what are called tabular numerals, which are monospaced so that they'll always line up in a column. Many OpenType fonts have both types.

That doesn't make sense. You can turn off the kerning.

No one who has ever had to do a substantial amount of typesetting would want to have to stop to turn on or turn off kerning just for numbers.
For display faces or non-tabular numbers it might be appropriate in some cases.
If you really want kern pairs for numbers, don't ever try and use the font to typeset an annual report or other document that uses columnar (tabular) numbers. It would look like shit to anyone who appreciates typography.

"The combination 74 seems particularly in need of kerning." What, and anything else containing a '1' does not?

George nails it. Per (sensible) default, digits are usually designed as "tabular lining", so there are no jarring differences in width between "11" and "22". A very simple example is that of two phone numbers above each other.

I had a nasty experience once with a Univers, as the version I had did have a lot of kerning in the digits, and suddenly all of my tables were off.
The reverse is also true: even with added kerning, these digits are no match for properly designed real "proportional" digits. A "1", for example, is typically designed to fit in the center of the width of digits, which means that it ought to be kerned both left and right against all other characters.

Proportional digits are explicitly designed to be used in contexts where horizontal spacing trumps vertical alignment, such as in running text. That also includes kerning: Robert Slimbach's Minion Pro contains "tabular" digits without any kerning, as well as "proportional" digits, which do contain proper kerning for your "74" example.

Andreas, what you're misunderstanding is that the numerals aren't just not kerned, they're not even proportionally spaced. This means that the sidebearings are especially wide on 1, for example. Simply turning off kerning on numbers that are still proportionally spaced still won't make them line up in a table. The fonts that you're using are probably TrueType or PostScript, which only has one type of numerals built into each font. Often these font families would have multiple versions, one for each type of numeral, but when Microsoft or whoever licensed them for bundling, they only licensed the font with tabular figures because their business customers use their computers for spreadsheets and such. The newer Opentype format allows several kinds of numbers to be bundled into the same font file.

If I want to have numerals line up in Microsoft Word, I use a table.

You're going to have a cell for each digit?

Look at the attached image. All of these examples are from the same typeface, Minion Pro, which has multiple types of numerals built into its OpenType features. If you want the digits to always line up, you use tabular numerals ("unkerned", in your parlance), but if you're setting them in text, you use proportional numerals ("kerned"). I also included proportional numerals with the kerning turned off, to demonstrate that simply turning off the kerning isn't enough to get them to line up.

What has this to do with kerning? The year 1874 looks horrible unkerned.

URW Palladio has proportional figures; the versions of Palatino, Times, and Garamond that you're looking at have tabular figures, which is why they appear "unkerned."

The two things are independent. You can have tabular figures that kern if you want. Just turn kerning on or off!

I give up.

You obviously don't understand what I am talking about, so don't worry. Kerning works only when kerning is selected, so you could have both. Duh.

It's possible to design a font to work this way. I had a client who required proportional and tabular numerals in a legacy application that didn't support OpenType features. I created tabular numerals and kerned every numeral and period combination. That way the client, in this specific application, could activate tabular numerals by disabling kerning. It's not the "proper" way to do it but it worked in that case.

One big problem with this technique is the decimal point. If you want it to be tabular, you end up a huge gap around the period in your text, which will need to be kerned against almost everything else in the font. It sucks.

Maybe another character slot could be used for a decimal point, instead of the period.

Yep. And they you need an alternate comma too, for stuff like 1,000,000. You'll need a monospaced slash, space, brackets etc. But then its no longer a simpler solution than using the accepted OpenType numeral feature. OpenType numeral features have been around for about 20 years. Maybe it's not that well supported but it works in applications that professional designers use and they're most likely who needs tabular. If you come up with a variety of solutions for the problem, you practically need to read an owner's manual every time you switch fonts. Like, this one font has an alternate decimal, this other one contextually swaps for another decimal and this other one uses OpenType. With the standard OpenType numeral features, a designer should be able to switch fonts and the tabular/proportion aspect will stay intact. If you think proportional lining figures should be the default, I totally agree but only because it makes the previews look better on font vendor sites.