At the moment, the Liberal Party and the Conservative Party can only nominate people of their ideology...as well as Moderates. The problem is that the Moderates are likely to lose if they run in an extremist environment that favors the C+ or L+ world. Since parties does appear to exist more for gaining political power for their members rather than just to mindlessly promote their ideology, is it alright to allow Liberal Party to nominate C Presidents, and Conservative Party to nominate L Presidents? Doing so could help to prevent the extremist C+/L+ politicians from coasting to an easy victory by deriding the Moderates and securing the swing L/C vote...
Well... these nominees aren't the pragmatic selection of a group of cigar puffing politicians sitting in a back room somewhere, they're the popular selection of the party's base. Primary voters will be pragmatic, but only to a point; they desperately want to rally behind someone that will bring change they can believe in, referencing Obama's 2008 campaign slogan. A Liberal Party primary won't produce a Conservative candidate because the Liberal Party voters won't get excited about a Conservative.
This is a flaw in the presidential primary system. With some kind of preferential voting system, you could get a Conservative defeating an incumbent Arch-Conservative, by getting help from the Liberals with their second/third/fourth choice. Perhaps this could tie in with the Election Reform topic. But with primary nominations, it's very unlikely, since the Liberals will run someone they prefer more, and yes, inevitably lose.
This is seen in a lot of lopsided House districts around the country; it's ironclad locked for one party or the other, and their opponents won't nominate someone who stands a chance, because to do that, they'd basically have to nominate a member of the other party.