Could you elaborate? Firstly, I'm not aware of any programming languages that has Lawvere theories as a first-class concept. Secondly, when I last looked into Lawvere theories I couldn't work out any way of making them into a first-class concept in any way that would make them distinct from monads.

(On the other hand Lawvere theories may be useful for modelling effects in type theories. That may be so (I don't know) but is an orthogonal issue.)