There is also the slight disadvantage that they have to burn drams of reagents every time they cast a spell using this method so they are burning nuyen, grated the cost is small and if they dont cast too many spells probably wont exceed what the street sam is gonna burn up putting various specialty rounds down range. $20 a dram means setting a limit of 6 is $120 per spell cast. If the GM actually tracks or at least enforces the PC needing to properly track his reagent expenses then that will add up over time. It also opens doors for the GM to mess with the PC if hes constantly exploiting the I have 5 Force 1 illusion sustaining foci as the talismongers can only harvest reagents so fast and it can be possible to use up the local reagent supply thus either driving up the price drastically as talismongers have to start importing the reagents and all the local mages start competing to out bid each other to ensure they have the reagents they need. You also can get into the mage starting to be easier to track since hes single handedly depleted the local reagent supply, people will start noticing and they will start wondering what hes up to that is consuming so many reagents.
One could make the "if the street samurai uses too many bullets, it will drive the price of bullets up" but no one ever does. Seems people only ever want to screw over mages. Odd, that.
But on your whole "the cost adds up fast" the difference between a force 4 sustaining foci and a force 1 sustaining foci is 12,000 nuyen. That is 600 drams. And then there is the karma difference in binding a force 4 foci vs. a force 1 foci, which is 6 karma, and at 2,000 a karma, that's another 12,000 nuyen or 600 drams. So the price difference is enough for 1200 drams. That's enough for 300 spells where a mage raises the limit from 1 to 4. And of course, then there's the fact that a force 4 foci is a big step towards foci addiction.
On the flip side, binding a single force 6 spirit takes 150 drams. Binding spirits is something mages are expected (by the setting) to be able to do on at least a somewhat regular basis. This takes away from your "mages burn through so many drams casting spells talismongers can't keep up" idea.