The type of deed to be used is specified in the contract; the title company doesn't choose.
Though I'm not a lawyer, my understanding matches Robin's: in short, that the Special Warranty Deed exposes the seller to less risk than the General Warranty Deed. Therefore, I assume the wholesaler is thinking that this is a better way to contract and would make his/her contract that much more marketable to investors even if just assigning the contract.
I routinely use Special Warranty Deeds, have never had a title company question it and have never had a problem with a Buyer accepting it once I explained it. (If a Buyer did have a problem, I'd use a General Warranty Deed; I'm never going to lose a Buyer fighting over this point.)
Question: does anyone familiar with the deed types recommend General Warranty Deeds because they can cite advantages not previously discussed? I'd always prefer to be called out for a gap in my understanding than be allowed to continue in ignorance.