Is that not being just a bit too pedantic and argumentative?
If you look at individual ships across the board you could narrow the differences down quite a bit but that does not make any given ship irrelevant regardless of how similar it may be to any other ship.
The Krait may be similar to the Python in at least some areas but it does not make the Python irrelevant nor is it a clone of the Python to the point that the Krait is a waste of space. There are sufficient differences to justify the existence of both designs.
Where the Chieftain and Challenger are concerned, personally I currently prefer the Challenger over the Chieftain on multi-role capability grounds. The Challenger is effectively on par with the Asp Explorer in terms of role usage with jump range being traded off against weapon power. The Chieftain on the other hand is more of a combat centric vessel. Not sure how the Crusader is going to fit in the mix but if I were to draw comparisons with the Federal ships...
Challenger probably approximates relatively speaking to a Federal Drop Ship (FDS) in the Alliance ship range
Chieftain probably approximates relatively speaking to a Federal Assault Ship (FAS) in the Alliance ship range
Crusader is probably going to be to the Chieftain/Challenger what the Federal Gun Ship is to the FAS/FDS.