[eru]

That's a very weird definition. Eliminating tail calls is fun, and useful. But not all that essential in, say, a lazy language.

Having first-class function values in the first place strikes me as way more important. Purity helps, too.

[Evbn]

How do you write a function to sum a list of a billion elements in Haskell?

What would happen without tail call elimination?

[eru]

With foldl', of course. If you don't have tail recursion elimination, foldl' would have to be provided as a built-in.

Summing up numbers is inherently strict, so you'd want tail call elimination for that. But functional mainstains, like say, map or filter are usually not implemented with tail recursion in Haskell, because that would be too strict and would break on infinite lists.