It's generally accepted that as a filter gets dirty it gets more efficient at filtering out particles. Obviously it will eventually get clogged and flow will be reduced to the point that the bypass opens. But filters are designed to last 15000+ miles so there is no reason to get anywhere near that stage. If oil is changed every 6 months at say 3000 miles then the filter is a long way from reaching the end of it's life even if it's left in and used for 6000 miles. If that 3000 mile filter is working better than a new one why would you change it ?
One down side is the old filter contains a small amount of oil that will not be replaced with new but there is always some old oil retained in an engine whatever you do.
Now if you want to get into an an argument that's even more esoteric there have been studies done that suggest new oil is initially not as good as old oil in terms of preventing wear. This is based on the premise that new oil removes the sacrificial layer on metal surfaces built up by anti-wear additives like ZDDP and it takes some time for the new oil to replace this layer. It's not a big deal but does serve to illustrate that changing oil too often might not be as beneficial as it seems .
I tend to take a middle of the road approach and consider that the oil change intervals dictated by car manufacturers are not arrived at solely in the best interests of engine wear. There is a commercial aspect to making cars look cheaper to maintain which is why MB at one time allowed 2 year service intervals. If I was doing 10,000 to 15,000 miles per year then I would definitely do as you do and change it more often but If I was doing less than 10,000 miles per year I wouldn't see any benefit in doing an intermediate oil change. The caveat is that the miles are not all short trips where the oil never gets up to temperature in which case the oil should be changed more often.
