Not quite so useless. Sometimes peoples who come from sophisticated but totally different worlds can clash, like Cortéz vs. the Aztecs, Rus vs. Mongols, or Robert Clive & the BEIC vs the Nawab of Bengal. Those cases are really interesting, thinking about who would win is a nice exercise.
Usually in history, when such worlds clashed, due to being totally unprepared both sides would try to improvise a lot but one would eventually gain the upper hand, exploit some advantage that the other is unable to counter, and it ends up being a world shattering defeat for the losers.
In the above case (professional legionaries from classical antiquity vs medieval army equipped with same weapons) I think the legionaries would rout the medievals. What after all are the medievals going to do with antique weapons? That's like asking, how would the US marines do if given wicker shields and obsidian clubs and forced to fight Aztec warriors. Getting sacrificed atop Tenochtitlans pyramid, that's what they would do. Given equal weapons, usually the side that has the most practise using them wins.
Same with the medievals. Take any Richard Lionheart or Tancred de Hauteville, take their their lances, longbows, damascus steel swords, chain mail and huge ass chargers away, give them Javelins, lorica segmentata armor, gladii and ponies with small saddles and of course they will not fight as well against an army that has been training with these weapons from youth.
A more interesting question: Who would win in a straight up fight on even ground, Alexander's phalanx or Richard the Lionheart's feudal army? Alexander has only mediocre bows and weak armor and their horses would be considered too light for frontal combat by medieval standards, on the other hand his hoplites are ten times more disciplined than a feudal levy and they have a lot more practise fighting together. One side would end up totally mauled but it's hard to say which one.