I've added a section to WFBattles.com showing commonly collected WFB army combos.The new table also shows which other armies each collector most wants, loves, loathes and fears and which other armies they are likely to have painted (use the dropdown box to select the info you want to see).
To read the table...
...take the army on the left, look across and the number you see in each column represents the % of members who collect the left hand army who also collect the army in that column.
As an example, in the table below 37% of Beastmen collectors also own a Br (Bretonnian) army.
For further examples click the "show explanation +" icon on the page. (high numbers are highlighted in red low number are lowlighted in silver)
A couple of interesting things I found on the "Owned" armies; Wood Elf generals seem to be the most frequent collectors of other armies, followed by Beastmen commanders. I reckon that's further evidence that both armies are underwhelming on the battlefield as players opt to collect other armies more often than other army collectors. Having said that, with regards to Beastmen it could be that it used to be packaged up somewhat with other Chaos armies and some units are interchangeable with the newer Chaos forces (such as the WoC warhounds, spawn, giants and the chariots which bear some resemblance).
It also looks like a lot of players can call upon their Orcs & Goblins and The Empire to fight their battles as they are the most commonly owned armies. It seems that I'm not the only one who started Warhammer with a bunch of frightened gobbos spearheading my charge into stolid and (in those days) completely implacable and stoic Dwarf wall of steel :-(