Fair point. I do think teams should be allowed the opportunity to "Refresh" their squad mid-season but the mechanics of the January window are extremely distruptive and are open to abuse.
You could actually time it that the January window finishes on the Friday after the 3rd round cup games - that would at least minimise the impact on most team's league form??
I agree with the OP - it should be shortened - lots of teams spend most of January in a state of flux (as we are!)...