We answer this question with an emphatic - YES!
A native mobile app is a very important part of a complete mobile presence for an online retailer. Recent studies have shown that 80% eCommerce consumers mobile shopping time is done through apps (Comscore). So, if you only have a mobile optimized website then you aren't offering your customers the best and the preferred tool for interacting and transacting with you through mobile. This leads to decreased sales.
Until recently the high cost of getting and maintaing a native mobile app made the answer to this question an easy, "No." Here at Apptive we have dramatically decreased the cost equation and made native apps affordable for any size business - making the answer to this question an easy "Yes!"