They probably decided that it was just a little too on-the-nose if they had Lucy flat-out quote Rose.

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!!'

(Aww, I had Rusty quoting you. Hee!)

On the other hand, you could be totally right.