but if a drug is off patent, it may be manufacture

but if a drug is off patent, it may be manufactured by a generic. therefore, what benefit would the original branded manufacturer have in the pursuit of a Phase 2 clinical trial which uses their off patent drug in combination with an untested drug?

Are you thinking that once the Phase 2 successfully completes, then the original approved medication can be re-branded and re-patented in that indication as combination therapy and then another 12 years are added to the patent life of that original medication?