Bernie Sanders' large loss in New York has brought the issue of open and closed primaries to the forefront. Bernie claims that he lost by such a big margin because independents and others couldn't vote in New York's closed primary. This was the case because in a closed primary only party members can vote. Bernie Sanders thinks this is a bad thing. I say it's the right thing.
The basic point is that the primary process is meant to elect a nominee of a party. It then makes sense that the people who decide who that nominee should be should be a member of that party. People that aren't members of the party should not have a say in choosing the nominee. Most independents look down on the Democratic and Republican parties and so shouldn't have a say in picking of nominees of the parties they scorn.
Open primaries have clearly altered each party's race for President. Independents helped both Bernie Sanders and Donald Trump to many of their early victories and helped establish the momentum for their campaigns.
On the Republican side, the hard core conservatives went for Cruz while the "mainstream" Republicans split their votes among the other candidates. If independents couldn't vote in the Republican Primary, we could be looking at Jeb Bush as the Republican nominee. Now to be fair, Trump gained momentum and got more and more Republican votes as the primary season went on but would he have even stayed in the race if he hadn't been the "winner" in the early primaries and caucuses helped by independents and Democrats that switched parties? Is it any wonder that he is having such a hard time unifying the GOP.
Hillary's big victories in liberal, Northeast states (which should have been fertile Bernie territory) with closed primaries are evidence that Democrats truly prefer her candidacy. This is further evidenced by Bernie's complaint that primaries weren't open. Unlike with Trump, Hillary's insurmountable lead means that the will of party members is being followed on the Democratic side.
Independents have their chance to make their choice in the general election. There have even been recent Independent presidential candidates who have run credible campaigns when there has been high dissatisfaction with the party nominees. However primaries are meant to nominate candidates who represent the views of members of a political party, not those who won't bother to affiliate with one. Closed primaries are the only way to make sure that happens. If independents want to make a difference in a party primary then register with that party, even if only through that election. If not, then be satisfied to let the people who accept the nomination at the Democratic and Republican conventions truly represent the members of their parties.