No matter how amazing the Revel Beryllium speakers may sound and no matter what Revel says in their marketing spiel, it is exceedingly unlikely that the sound quality is influenced to any significant degree by the use of beryllium for the tweeter dome. Substantive differences in sound quality, for the beryllium speaker vs. a similar speaker in the Performa3 line, are the cumulative effect of numerous design differences between the two speakers. With respect to the F328Be this is obviously true given that in the Performa3 line there aren't any speakers with three 8" woofers. But even if you compare the F228Be to the F208, various differences will be found that will be much better explanations for any differences in how they sound, vs. the difference in the metal used for the tweeter dome.
The use of ultra-exotic materials in loudspeaker design has long been a successful practice, but more from a marketing perspective than an engineering perspective. Revel is no more and no less prone to this successful marketing practice than any other loudspeaker manufacturer. People (audiophiles especially) are impressed by the use of exotic aerospace materials in loudspeakers. Relative to a given stiffness, a dome made of beryllium will be less massive than an aluminum dome. A somewhat more powerful motor is therefore needed with an aluminum dome. This implies either a stronger Bl value or else greater wire length in the gap, either of which also provides the additional damping required by the more massive aluminum dome. All in all the aluminum dome tweeter will be slightly lower in efficiency and sensitivity. Beyond this, there is not likely any significant difference between beryllium and aluminum, as it applies to loudspeaker tweeters. I doubt whether anyone designing a speaker has encountered any difficulty finding tweeters with adequate efficiency and sensitivity. In general, tweeters are several dB more sensitive than the other drivers and need to be padded to bring their sensitivity in line with the other drivers. This is typically true even for tweeters that use non-metallic domes.
As to the question of whether there is any moral dilemma over the use of beryllium, the best answer is simply "no". The use of beryllium in aerospace and other industries is nowadays fairly common. And even if the use of beryllium weren't common, there are lots of other commonly used materials that are vastly more hazardous, that most everyone simply accepts as a fact of modern life. The common and unavoidable practice of storing fuel and highly toxic chemicals in facilities located within reach of urban centers poses a vastly greater risk to public health.
If what you really want in a speaker is for it to sound as correct as possible, objective measurements are the only way. Something that isn't pointed out nearly as often as it should be is that whenever there is a debate over which speaker (among several) is more correct, there is just one way that a debate of this sort can be settled, which is by comparing the objective measurements. Even if 90% of listeners prefer the sound of speaker A over the sound of speaker B, the only way to ascertain which speaker is more correct is by comparing objective measurements. If the objective measurements are so similar that it isn't apparent which speaker is more correct, then there isn't any genuine reason to think that either of them is more correct than the other.