Personally I would not use drivers like these in my main system but I think larger drivers would work out O.K. for a secondary casual system, like the NOS (new old stock) Radio Shack 5.25" "full range" drivers I have been playing around with. Highs are a bit muted but are pretty smooth and midbass is decent and IMO like the FR drivers in better table radios like those from Tivoli and Boston Acoustics, while I believe they are not highly accurate they are easy to listen to for a long time.
I can see why there is a whole area of DIY speaker building dedicated to building FR-based speaker systems, using drivers from three inches all the way up to eight(!) inches, though 4 and 5 inches seems to be the "sweet spot" for most FR fans since these can produce acceptable bass if the proper enclosure is utilized. The other signficant reason FRs are favored is the fact there is no crossover which, if it is not very carefully designed, can cause audible sonic artifacts.
For a lot of FR fans though, they use a subwoofer to augment the lowest bass and since the xover does not occur in the midrange spectrum, that avoids affecting those important frequencies. And for larger FR drivers, many use a "helper" tweeter, usually crossed over way up at 8 or 10kHz again avoiding the midrange band. A simple 6dB per octave xover is usually used at such a high frequency, commonly in the form of one capacitor to minimise xover artifacts.Fostex
is a very popular brand of FR drivers.
Parts-Express sells many FRs, including budget stuff like this 8" driver
many pair with a inexpensive soft-dome tweeter.