Actually (and I obviously could be wrong), but I think it may simply have to do with the love of puns, play on words, etc. in the dialogue. Lucy is specifically talking about being with the Master, ie, marrying him, thus the "for better or for worse," quoting traditional wedding vows. That just so sounds like something that Rusty would grab onto. The intention was probably to very clearly draw that parallel with Rose's line, but then he jumped on the 'Ooh, I can get the point across AND make a cool play on wedding vows! Go me!!'

Oh, no, that intent makes a whole lot of sense. Though, if you then backward parallel again, it makes Rose's 'forever' and 'I made my choice a long time ago' sound even more vow-y.