I was told by my dealer that JM Lab voices all its speakers for use with the grill on. In practice though, when I took the grills off, the sound opened up, detail improved, and the soundstage widened by about 6 feet! So I don't buy that claim.
I think manufacturers say these things so that people evaluate a product's aesthetics with the grill on rather than with the grill off. Most non-audiophiles prefer looking at grills rather than drivers. The WAF factor, maybe.