Below is a good video example of the utility of jiujitsu for professionals. This security guard takes down a wilding attacker and effectively controls him using the “knee to belly” pin, or uki gatame (浮固).
Note that he is able to monitor both of the man’s hands, and his surroundings. Should the security officer decide to disengage, he simply needs to stand up and away – in environments where weapons and other concerns are present, or where there is no need to detain or control a subject, this is often the better course of action. Though not a solidly dominant position in jiujitsu, knee to belly is positionally advantageous. It’s advantage lies in its mobility and the ease with which you can transition to other methods or positions, or disengage.
When an encounter is not one-on-one, is not based on positional dominance-to-submission, and where situational awareness is of no concern – in other words, like when playing jiujitsu – things like knee to belly are often more tactically sound.
On the other hand, where positional dominance is warranted or a necessity, transitioning to a mount or other dominant top control would be in order. In jiujitsu, and judo if you want to go to groundwork, the nature of the sport aspect of the game demands this, as knee on belly would not be a strong position against a skilled opponent.
But we must remember the two things are different.
This is particularly true if as an instructor you are teaching self defense or police or security professionals. Take care not to conflate competition methods with controlling or combative jiujitsu in defensive or tactical encounters.
The art is big enough for more than one approach. Apply common sense to what makes sense under varying conditions.