When you think about it, its a powerful ability: you don't see the true range of your opponents sword, the slash angles could be tricky and difficult to predict as well. Oda specifically chose this ability to not overshadow Shiryu's swordsmanship, but enhance it only. If Oda gave some Zoan or another ability like Logia, it would overshadow his swordsmanship, but with invisibility, Shiryu can combine his swordsmanship perfectly, or even use faster undetectable attacks as we him vs CoO users Koby+Garp.