Because thats pretty much the only reason I need it, since im getting 99 mining at LRC.

I didn't get it till way after I got 99 mining, so I have no data involving it, but I'd assume that if you're fairly close to the requirements you wouldn't lose very much time getting it. Also, you'll probably want to finish all the diaries at some point, so do this one now while it can benefit you.

varrock 2,3 and 4 are worth it alone for battlestaffs.

 

but yeah it is for mining. The chance for double ores is just over 10% according to rswiki.

So that's about a 10% increase to your xp rate.

I'd say it's worth it. Speeds it up a decent bit, plus all the extra benefits.