If one were to draw a Venn diagram showing how the player bases for SFB, FC, Starmada and ACtA:SF overlap, you'd likely find more than a few people into some, but who may not necessarily be overly keen to rush into any others; but even so, I would like to think that each circle of gamers would still be large enough for the various SFU tactical starship combat games involved to each succeed on their own terms.
(For FC in particular, I would note that it has evolved over the last several years; but it was never intended to be the same game as SFB. Although, in terms of game mechanics, it does lend itself to more of a direct comparison; since both systems are more closely related to each other than either are to the Starmada or A Call to Arms game engines.)