Only get one if your serious about racing x-country or othe long races.
Just a side note, its kinda crazy and dangerouse but Ive read about this procedure of sealing off all openings having a air compressor nozzel attaced to the gas cap, then heating the tank (empty and washed out first) with a heat gun, after that pumping the tank with compressed air. This gave the guy some extra room for gas, I cant remember how much though.
I would not reccomend doing this. Matter of fact Ill say DONT DO this.
IT was just an interesting story I thought I would share.