IMO it is perfectly legitimate and desirable to engage in metaphysical speculation. Ideas that are metaphysics today might very well be established science tomorrow. In fact, I think that science is driven by speculation about the unknown. Claiming that your metaphysical ideas are absolute truth however is verboten.

James Clerk Maxwell:

“It has been asserted that metaphysical speculation is a thing of the past, and that physical science has extirpated it. The discussion of the categories of existence, however, does not appear to be in danger of coming to an end in our time, and the exercise of speculation continues as fascinating to every fresh mind as it was in the days of Thales.”