You say flat - I say accurate reproduction. If we follow a simple logic the job of a reproducer is to output a signal as true to source as possible. It gets more complicated with loudspeakers in rooms, but it holds true nonetheless. If you are listening through neutral reproducers, and are not happy with what you are hearing, then something else is wrong. If you've taken care in setting up your system using EQ below the transition frequency, and there are no obvious problems in the rest of the chain, then we must conclude it is often the program material (often mixed/mastered using inferior or inadequately calibrated equipment) that is to blaim. As F. Toole writes: even the objectively best loudspeakers cannot sound the best all the time.
In my opinion this is not an excuse to continue using, in my view, inferior products with resonances, irregular directivity, timbral balance issues etc embedded in them.
The current testing shows in fact that all of these preferred differences have to do with treble and bass levels, which is a feature available on amplifiers since many years, and even then we don't know how much testers were compensating for the recording, their hearing, or the loudspeakers. Resonances have always found to be detrimental. In short, from a logical perspective there cannot be a valid argument against accurate loudspeakers.