Airlines will book you together if seats still remain together...They're not gonna split you up just for the fun of it.

Only way to guarantee seats remain together is to select seats ahead of time. People complain about "junk fees" because they're mandatory parts of the cost. If a website is upfront about a benefit you're losing, it's not a junk fee.

Lastly, these basic fares are really designed for single travelers on a tight budget with fixed plans. Delta's website makes this clear, and websites that don't make it clear really should.