Yeah, that is exactly how we feel about it. Enough things can go wrong that you don’t need to add this when you face a problem 
But, if money was really tight, I would feel differently. We have also tried it and did not run into any trouble.
Although the problem is slightly different if you always recycle the old insulin, since you start accumulating very old insulin, in small amounts of course. That fact could create a problem, not in terms of the % of old insulin (it still would be very small), but in terms of the kind of organic chemistry reaction you could see. For instance, with oil, if you add a minuscule amount of rancid oil to a flask of new oil, in a very short amount of time you will see it all turn rancid (the principle of chain reaction, very common with organic compounds).