I think a focus is integral to playing a magician. Having a Sustain Focus and a Weapon Focus should not be something a GM should penalize the magician for unless they also drop the bomb on the Street Sammy with her .85 Essence total every chance they get. They are part of the game IMHO.
Gaining Focus Addiction During Play
When a magician relies too heavily on foci, the become a "crutch" that begins to undermine the magician's abilities. Prolonged (ab)use of more foci than the magician can safely handle can burn out their Talent.
Focus Addiction (p.26, Street Magic) uses the Addiction rules (See Addiction quality, p.80 and Substance Abuse p.247 Shadowrun, Fourth Edition). The gamemaster can call for a a Focus Addiction Test at any time she feels the player is abusing foci; a good guideline is whenever a character has a total Force of active foci in excess of twice their Magic attribute...
from Digital Grimoire p.8
I noticed you mentioned your GM doesn't care much about the "rules" but decided to post the source anyway since I am home and had a few minutes.
I don't know about you, but I'd rather have it now and use that Karma to get myself on the track for Initiation for Masking, or Centering!