It would only have been better to signal that we were prepared to leave without a Deal if that was actually our position. By definition, given that Parliament is ultimately the decision-maker, it is not what we're prepared to do. There are, I would suggest, maybe only a dozen MPs who are genuinely prepared to leave without actually arranging a deal, and the rest are only interested in *pretending* that they are as a negotiating tactic. There is probably wider support for No Deal in the country as a whole, but that's unlikely to ever be tested.
I think you're looking to the wrong amendment anyway. As your question states, Parliament can say whatever it likes about No Deal, but if it's the default then if they want to avoid it then they actually have to *do* something active to achieve that. The vague statement "that this House does not support leaving without a Deal", or whatever the precise wording of the Spelman amendment is, carries no such weight. On the other hand, the Brady Amendment -- the one that the government and leading Brexiteers backed -- that actually *does* send a signal that the House doesn't want a No Deal exit, because they are chasing changes to the deal in order to avoid this instead.
Whether or not those changes can be achieved is another matter, but make no mistake: The Brady Amendment undermines No Deal equally as much, if not more, than the one you're complaining about.