Without romanticizing and glorifying Grunwald and the golden age of the Jagelonian dynasty, I don't think Poles would have maintained their national identity through the centuries when their country ceased to exist as an independent polity.
Part of the reason Sienkiewicz wrote his novel on the TK was to inspire Poles to hope for a day when they would get their country back.
Conquered and subjugated peoples need a glorified, romanticized history in order to survive as a people. That romanticized (and distorted) vision history typically remains with them even after they regain freedom. That is why the struggle with the Teutonic Knights remains so prominent in the Polish psyche today, and I don't see anything wrong with it.
If that romanticized history inspired hatred of some present-day ethinic or religious group, then there would be some reason to worry about it. In this case, however, the only hatred is of a defunct religious order. I don't see the harm in hating people who have been dead for hundreds of years.
Germany never went through a centries-long dismemberment by foreign powers like the Poland went through, so I can understand why the phenomenon is not present in your country.
But still poor, isn’t it? :)
Poles and their self pity...