Thanks to Jun for his interesting, complex model about thresholds in Japan's Diet (especially HOC); but it is not necessarily true that hybrid systems do not have a threshold too. In NZ's parliamentary elections (MMP) for e.g. [from the official electoral web page] .:
"Your party vote is important because it helps decide the share of the 120* seats in Parliament that is allocated to each political party. If a party crosses the minimum 'threshold' (either by winning at least one electorate seat, or 5% of all party votes) it has seats allocated to it in close proportion to the percentage of party votes it receives."

Similarly for Germany's hybrid system (MMP):
" The electoral law stipulates that a party must receive a minimum of 5 percent of the national vote, or three constituency seats, in order to get any representation in the Bundestag."

There is also a threshold in the Ukraine's MMM (Mixed Member Majoritarian) systems like Japan's where the two tiers of SSD and PR are separate and no adjustments for the vote in PR as in MMP (Mixed Member Proportional) systems like NZ and Germany.

So many hybrid system democratic parliaments are elected with 5% thresholds and the intent is to exclude micro, one-issue, or extremist parties. Interestingly, the European Court declared Germany's imposition of a threshold (5%) in EU elections to be unconstitutional according to EU law exactly because it discouraged small parties.

FYI.
Best,
Ellis