You dont even have to go with the fluff for the given armies to explain it. You can take tomp spyders, strap some big guns (annihilation barge guns?) onto your back and then call your tomb marauders that use the broadside rules. The allies opens the door to two different options:
1. You can take your 2 favorite armies and (potentially) run them together to fullfil some sick, sadistic dream you had.
2. Convert the current models of your current army so they are still "that army" but use the ruleset for one of the allies listed.
As pointed out in my post about pariahs, I'll be running them as grey knight termy rules (all hammers). I call them my pariahs, led by a pariah lord, all on 40 MM bases to keep to the rules needed. Granted, I will need to use GS to beef them out a little so they feel worthy of a 2+ armor save, but the conversion is awesome.