There would be no point until they released Imperial guard, there are no tanks worth looting at the moment.
Because there was never an Eldar, Space Marine, or Chaos Space Marine tank ever converted. They don't only need to be Leman Russes. Orks take bits from any army they can get their hands on. A little variety is always nice.
Deathskull lootaz do, but the Bloodaxes that I was referring to specifically trade with the IG. In the earlier IG codices you could include units of Bloodaxes and in the Ork codices include an IG tank for each Bloodaxe unit you had on the table.
In the newest edition of the rules Imperial Guard are allies of convenience with the Orks, so you can field Imperial Guard tanks or have a squad of sneaky gits. I didn't know that about the older codices, also. I have the most recent Ork codex.
I used to have both an Ork and IG army under 2nd edition because they could share tanks and units such as Ogryns mercenaries.
