It doesn't really change anything to their inherent value that you build them from the hypotheses up or from the conclusions down. It holds the same that this set of interactions lead to this behaviour.
Sometimes going down can be equally interesting: "What would a system where tariff improves the overall well being of a country inhabitants look like?". Contrast the model requirements with empirical data and you have an interesting paper.
The non-mainstrean economists (aka heterodox, aka cranks) are usually more obviously pursuing an agenda, but they definitely usually don't have better empirical evidence.
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Mate, this is not physics. This is not a field with a measurable reality where you can, in fact, say that those who reject the empirical evidence are cranks. Economics allows for multiple incompatible interpretations of the same empirical observations.
Economics has several characteristics that make it unlike other sciences: basically the "underlying observable" is not an objective thing with an independent existence; unlike an atom, the object of economics changes when it's being studied and changes with the underlying changes in political and economic structure. Even its aims are open for discussion! Economics has more in common with sociology or philosophy than any of the hard sciences.
It's easy to see, if nothing else, because economic theories fall in and out of fashion just like political ideologies do.
I think it's perfectly fine to disagree with non-mainstream economics, or to criticise it on some concrete grounds. But to pretend anyone who disagrees with $current_view is a crank... Well it's symptomatic of the quasi-orwellian reality which certain sectors want to impose: it's not enough to say "ours is the right way", they want to say "ours is the only possible way".