The significant percentage savings are only attained while at cruising levels and speeds. In the climb and descent the winglet has a negative effect. In addition it is extra weight which, as gross weight stays the same, has to be taken from fuel or payload if required.

On the Boeing 747-400 winglets were standard, except on the ones used by Japan Air Lines and All Nippon on domestic flights within Japan where calculation showed they had a negative net effect.

The winglets are not necessary for flight and if damaged on the ground (and of course they tend to get hit first) can be removed for flight, there is a standard procedure for this.

Anyone looking for inspiration for them has to go back further than Rutan. Eagles and similar birds have been doing this for a lot longer, with those tip feathers they spread out while cruising along. The reason they do this is exactly the same.