it depends on how much business your doing on ebay. i was selling several hundred items a month before i opened a store but kinda wish i had done it sooner. there are slight breaks on shipping and fvf and listing fees but not enough to add up to anything significant, especially if your not doing volume and if i remember corectly in some cases not having a store is better in that sometimes regular ebayers get 500 free listings and things like that where as you dont get that in stores.
in my opinion the biggest reason to have a store is for repeat business and to market to your customers. when you have a store you get all your customers contact info so you can contact them outside of ebay and try to sell to them outside of ebay without the ebay fees. also you can send out newsletters and notify people when you have new product, you can set it up so your customers get reminders to leave you feedback after a week if they havn't done so alreayd, there's a lot of cool ways to automate your ebay business and market to your ebay customers. one other feature i like is there's added search functions as opposed to have to look page by page through your sales for stuff.
for me a store works great because i have a consistant product so i am always selling the same thing, my buyers are return buyers. selling random stuff from storage auctions note quite as worthwhile in my opinion unless you have a niche of things you tend to get because your buyer who buys the stereo you found in a unit probably won't be interested in the legos you found in another unit.
i would say if your doing 50plus items a month a store is probably a good idea