There are a number of high end preamps with dedicated line inputs (NOT DIs) - Crane Song and Buzz are 2 off the top of my head. Most preamps have at least 2, and sometimes more, gain stages, and the line inputs usually feed the circuit right after the 1st stage, so the signal can benefit (or suffer detriment) from the rest of the circuitry. A DI input should have the equivalent of a passive DI box built in and feeding the 1st gain stage, taking advantage of all the subsequent circuitry. The primary purpose of a DI (input or box) is to provide proper impedance matching for pickups, which have a significantly higher impedance than a microphone or line level signal.
A secondary function of DI boxes (though not necessarily inputs) is to balance the signal to avoid losses in long cable runs - e.g. from stage to FOH mixer. Most preamps (even high end) are unbalanced internally, and provide balancing at the output, often using a transformer. Many people like the 'sound' or 'color' of particular preamps, and so might use them to inject line level signals to the next piece of gear in the recording chain.
Some preamps do have a dual purpose line/DI input - the better ones are usually switchable between functions. In ones that aren't, the input impedance is usually a compromise between the best for line and the best for instrument - the sound may suffer a little on one and/or the other application, but only your own ears will tell you if this is a 'deal breaker' for your purposes.
I give up. In fact, I'm gonna fire off a letter to Mr Fearn! I paid damn good money for a line to mic level converter, when here all along they are the same! Dammit. 
