Actually, there is such a requirement in the practice of science. Particularly when there is no sound argument against the currently accepted theory.
It sounds like you are overstating your case somewhat there, JS. One can question a theory until the cows come home without an alternative theory, under certain conditions, and still be practicing science.
Some of these conditions might include:
One has new experiemental evidence which appears to contradict the theory
The theory being questioned is new, and one is questioning the mathematics, the predictions, or the design of the experiments which support the theory
"Less-filling / tastes great" debates such as nature vs. nurture, or low-carb diet (Atkins) vs. low fat (Ormish)
And of course one is always free to look for logical flaws or faulty predictions, even if one has no replacement.
Despite some posters' intellectual vanity, it really is acceptable to say "I don't know" or "I haven't studied that yet" or "Gee, how does that model work in regard to XXX?"
Full Disclosure: It may be true that none of these examples apply to the typical posting by cre-o's here. But there is stil such a thing as refining a model; or such a thing as poking holes in a model, in order to alert people it's time to work on a better one--even if you haven't worked out the improvements yourself.
Experimental evidence contradicting a 145 year old theory will have to be pretty powerful. A number of hypothethetical examples are posted on most of these threads, but no such powerful contradiction exists for evolution. Any theory as vast in its implications as evolution will spawn smaller explanatory theories and hypotheses. These are fair game, and rather frequently revised.
Evolution is neither new nor faddish.
In fact, the word "theory" is generally applied only to well established ideas, not easily overturned.