Occasionally, you come across a random post where all answers except one have received a down vote. Moderator wants to rule out foul play, and tries to do an IP cross reference, only to find that the IP data is only for the site they're on at the time, and doesn't include the data from the parent.
For example, On a stack exchange site, you might see an IP history of:
xxx.xxx.233.176 262 xxx.xxx.201.77 209 xxx.xxx.201.127 155 xxx.xxx.236.126 117
But on meta, for the same user:
xxx.xxx.233.176 56 xxx.xxx.201.92 38 xxx.xxx.202.153 18 xxx.xxx.202.22 18
I believe it would be useful for the IP tools (history and cross reference) to be combined, perhaps with a breakdown as to how much of that activity is on the current site (there might even be scope for linked accounts to show as an additional column)