Well 5% may not seem huge, it makes a diffrence, a big one.
a lot of it is in terms of how you do your dammage. Take for example if you were a rouge and you used a set of daggers with a super fast Attack Speed then +5 dammage could be more than 5% and you are dealing that extra dammage each stab.
if you are not a rouge or have a slower weapon then it is not as usefull... note i say AS usefull...still better than the soward that only dose 100-150, i will take one that dose 105-155.
Their are other enchants, like ones that add Agil or Resits or absorbe, the list goes on. I assume that you are relevitly new to the game, Every thing in this game is about nickeling and dimeing stuff. You wont get 300 added to you agility form one item and you wont get a extra +50 dammage form a enchant. You may have a total of 80 added to your agil... but not form a single item, that said their are multipule ways to raise you dammage and you cant count on one enchant to do all the work.
The biggest underlying point is it is better than nothing..and i will take any help i can get if that is 5 dammage for a striking enchant or 4 agil on a glove, it is more than i had without the enchant... pricey yea...but 5 g dosent sound like much to me... i have used my soward over 800 swings and that equals 4000 points of dammage, in the long run... thats worth 5 G especialy when ya think... i am going to keep using the blade. When ya hit level 60 and have Verry nice equptment and their is none better enchanting is a way to get more out of what you already got.
Hope that helps