Hi Bencemac,
can you help us with letting us know the usecase that you would like to solve with insource:text? What are the situations that you would like to use insource:text an how often do they appear?
Apart from performance issues, we are also hesitant to add more fields to the form since it increases complexity. Especially if users are not too used to the intricacies of the wikimedia projects, we don't want to overwhelm them with our search form. But we try to find the ones that are most useful to all, and to make sure we completely understand the benefit of insource, your explanation of when you use insource and why will help a lot!