I do have precisely the problem you describe, in that the quality of the practice is limited by the ability of students to present competent punches to for their partners to defend against. Some of my students have a background in striking arts and others do not. I already have enough to teach on the grappling side of things, so I give just the bare minimum of instruction in punching to those students without a background so they can present the correct stimulus for the techniques being trained.
It's not ineffective BJJ - everything I teach is stuff I've done in sparring against high level strikers. But the students are only going to get as good as the training partners they work with. So I tell them that they are learning the basics for dealing with untrained opponents and that in order to get really good at grappling against strikes they will need to spar with more experienced strikers - which typically means visiting the MMA class. (Although sometimes we have enough competent strikers in my BJJ class that we can get in some good sparring sessions for everyone.)
I have the same situation... except instead of the MMA class, we only have a Shotokan Karate class to send them to for striking.
I look at this more as an opportunity than as a problem. When I have a student throw a punch at me, to demonstrate a technique against a punch.... I have to to the technique with the punch being thrown, not with the punch I wanted. I have students that have various types of previous / simultaneous training. Karate, TKD, Boxing, MMA, Muay Thai, Kung Fu, Kali... I also have students who have just been in fights at the bar and some that have no clue how to punch at all. When I know the student, I have some idea what to expect in terms of the punch I will receive. But, when I demonstrate the technique against the boxer.... sometimes the kung fu guy will want to feel it... and then the bar fighter will want to feel it. This lets me train my technique more in depth and shows me where I need work.
The most interesting is when we have a new student. Then you truly do not know what to expect when they punch. (yes, I do talk to them and ask.... but I have had 3-4 people showing up with "a few years of BJJ" that were completely lost when we did our shrimping exercise during warm ups and we have had our share of "karate guys" that would be knocked out by a wet paper bag with limp noodles for arms...) Getting the technique you want, with control, against a punch that you truly do not know what to expect.... is pretty decent training. Sometimes you find that trained strikers are easier to deal with than untrained flailers.... especially when you have to maintain control of their safety as well.
This training has worked pretty well for me. When I go to other schools or spar with people of different backgrounds... the experience of getting my technique from the punch thrown has been really beneficial to getting my stuff to work in sparring sessions with other arts. That said, you do have to get out of the house as Tony says, and try this stuff with people who do know how to punch. If you stay only with in your little group, you will be limited by skill set there. But, if you can get out to different groups, you can use your experience of getting different types of punches from different people to help you improve your techniques. So I see this as an opportunity, provided you augment it with real sparring, with folks you can hit you. Interestingly, if you do get attacked on the street.... you are as likely to get a completely untrained guy trying to knock your block off as you are to get someone with training.
