My I.I. practice with R. (my son, more verbal) has always lagged behind that with M. (daughter, less verbal). I think partly this is because I harboured doubts about the applicability of I.I. to a semi-verbal child for a while. With M. we started from a place of reduced eye contact, reduced connection, and few words, so I.I. yielded quick and dramatic results (and was very satisfying as a result). It was easy to see that it was the right thing to do. With a semi-verbal child it's a bit different: their scripted speech (echolalia) might give the impression of being very advanced and so you can start to worry that I.I. is actually taking them back a step.
Example: R. can recite many car related facts: 'Mummy's car is a Volkswagen, Daddy's car is an Audi, Mummy's car is black, etc.etc.' (and so on for all the neighbours' cars as well!) This is the kind of scripting that makes people who don't know him well express surprise at how he could possibly have SEN when he is such a 'clever boy'. We also have videos of him apparently 'reading' entire books, which looks really advanced for a three year old - until you realise that I have just read him that book beforehand and he is recalling the entire episode (including the inflections of my voice, the 'extra bits' I added in to the story, etc.) just like a voice recorder.
In a video-recorded session of I.I., however, he appears considerably less verbal sometimes. An outsider's observation might be that it was encouraging him to go backwards. We play games (often with phonics sounds because that's what he's studying in nursery) - for example: we say 'c-c-c-c-c-car!' and copy each other saying it in funny voices and have a good laugh about it. Superficially, that seems less advanced than reciting 'Mummy's car is a Volkswagen'.
However, I have become convinced by the I.I. literature that although linguistically and syntactically it may be less advanced, communicatively (pragmatically) it is more advanced. 'Mummy's car is a Volkswagen' is actually just the repetition of a series of sounds that I taught him to say - whereas playing funny games with sounds demonstrates reciprocity, turn-taking, pleasure in another person's company, humour ... In short, lots of the 'Fundamentals of Communication' in I.I.-speak. These 'Fundamentals' need to be reinforced if all the clever speech is to become meaningful, functional and actually cognizant of the presence of the communication partner!
I also think that as I become more proficient at 'semi-verbal I.I.' the goals of reinforcing the 'Fundamentals of Communication' and extending his verbal development will not be in opposition but will actually complement each other nicely. For instance, a few days ago we had a really satisfying session with a selection of drums and vehicles where we experimented with different combinations (the car is on the drum, the tractor is on the drum) and sang a song about each combination. R. was getting enthusiastic about making up new verses to reflect all the different possibilities - it was the most flexible use of language I have ever heard from him.
So in summary, I wrote above 'you can start to worry that I.I. is actually taking them back a step'. I think it's true that it does, and this is exactly what needs to happen. If good verbal development continues without a strong foundation in the 'Fundamentals of Communication' you get an individual who can talk at length, monologue-style, about their own special interest without awareness of the effect on the listener - and so I think 'back a step' is completely justifiable to encourage speech which is flexible, functional and socially aware.