True Masking by itself is only useful to 'go naked' (or to hide initiate status)
You might now Mr. X is a magically active (he has foci) but atleat from my reading, masking could make him appear to be an almost burned out worthless mage, when he is really a high grade initiate, etc.
Likewise it is an interrpurtation but gives masking a little more punch.
the various 'metamaigc' technicque (especially the 'shielding' ones). While not readily apparent until the are used, IMO, it is noirmally fairly obvious that they were used. (something interrfered with your mana) and without masking, to me it is also obvious if anything 'funny' happened (the mage abosrbed some of it, obviously if it is reflected back' that is noticable).
WITH masking, unless you make a perception roll and peirce the masking as normal, all you notice (to me) is standard counterspelling UNTIL somethign obvious is wrong (such as a sucessfull reflect).
A little efffect but still adds a little more bang, but really masking by itself is really only for doing the full pretending to by a mundance, which means not having any magic...
Now by the time you can get improved (grade 2), being a 'mundanne' with 2 'hidden' magic effects active can be quite useful.
So you can smuggle two foci.
A sustain foci and an active spell. (hmm physical mask, go to a meet as a mundance who DOSESNT even look like yourself)
no foci but two active spells (either sustained by you, with penalties) or spirit of Man innate spells.
Being a 'mage' with two hidden 'magics' and the world thinks your jack mundance can be a big benefit if a covert syle.
Heck even being a seen as a 'mage' rather then an initiate with a power foci 6, is a pretty big help....

(if you manage to get yoru hands on one... Extended Masking is the first step in keeping one... you walk around with OUT it masked and every mage is town is going to aiming for it).