I think this is a way for USCIS to do backlog reduction..Biometrics will be taken of you now so you they dont have to taken after you do your stamping in my opinion...CAn anyone check for the "code" on your biometrics request? IT will be either 1 2 or 3.
Here is more info on USCIS biometrics:
On most occasions the system does not show the completion of biometrics on I-485 applications. The receipt notice contains a number (1, 2, or 3); depending on the type of biometrics that should be associated with each Form type.
Biometrics and Fingerprints are needed for the I-485, but they will not be entered under the I-485 receipt number because the biometrics can be used for more than one application.
If an applicant did not have biometrics taken for each receipt number, he/she can set up another appointment with the ASC (the applicant will not have to pay another fee), or the adjudicator may send a Request for Evidence (i.e. for fingerprints).
So what this is saying is that maybe in future, Travel Documents, IAD etc will have Biometrics associated in them...SO USCIS is getting yours while they give you your GC...That way they dont have to give you your GC and then a year later ask you to come back again for biometrics..In my opinion..