That's a good observation Andrey and not too uncommon for database literate folks to remark when they get a bit deeper into SAP BW.
The main point here is that the comment with the DMUID is generated newly for every different SAP BW query. That means the DMUID will change when the rest of the query changes, which in turn leads to the same number of new SQL queries than without the comment.
So, disabling this wouldn't enhance the usage of the SQL plan cache.
BTW: the plan cache is never ignored, disabled or not used. The SQL queries are put into the cache and it can be used to analyse issues, e.g. performance problems on DB level and link the SQL queries back to SAP BW terms.