I use Lantus SoloStar / Humalog Kwickpen pens and only change the needles when I change the pens. I have always done this for the past 30+ years and have never found any problems with infection or pain etc. When you quote that .. "they are designed for single use", I'm not sure where that came from since it is quite clear that they can be used multiple times with absolutely no problems providing you are careful in handling them. Obviously in surgery and hospital use it has to be "use only once" .....
One advantage of multiple usage is that, in order to clear any air bubbles, you only have to do that once when the needle is added, so this ensures good dose accuracy with no chance of bubbles reducing the dose. On average I use about 3 to 4 needles per month and the last box of 100 needles lasted me over 2 years. Same with finger pricking lancets .. I only change it about once/month (~ 200 uses) and have never had any problems what so ever
But I appreciate no everyone may have the same experience.