Gerrymandering would be less useful if fewer people voted along party lines, but not useless. Any demographic edge is better than nothing; even if only 5% of voters decide on the basis of party affiliation, that's 5% more than nothing.

In the normal case, wouldn't a person usually vote in the next election the way they voted previously? A party's platform is generally the same election-to-election, and if I voted previously for that platform, I'd tend to continue voting for it unless I had a pretty significant change of heart--and then changed it again for the election following? Unless either the party or the voter swung wildly around, gerrymandering doesn't require "blind" allegiance to a party.

The issues favored by one party or another change dramatically over a period of even a decade. "Party" is just a way to make people feel like they're part of a team.