QUOTE (Shinobi Killfist @ Aug 8 2013, 06:29 PM)
Yeah that is one of those areas that I'd never allow as a GM. It breaks sustaining focuses and addiction rules hard. The force of the spell is irrelevant for far too many spells that you would sustain allowing you a cluster of effectively force 6 focuses for the price of a handful of force 1s. Even under 4es background count rules I would not allow it as it means I either let the mage wreck the game or I shut them down with background counts. Swinging between the 2 extremes is not fun IMO.
Well, coming from a SR4A table that both allows it and uses Background Counts consistently, it is really not all that broken, as it takes an Edge use to do it. There are enough Wards and BGC that it is a fool's choice, though on occasion is useful. Never use it myself, personally (Edge of 3 is too valuable for stuff like that), I just use combo Foci, at Force 3-4 to make it work. Even in a BGC 2, the Force 4 Sustaining Focus will let you get 2 passes, assuming you max out your hits into the Focus (my preferred method, actually).
The use of Reagents in SR5 is an interesting option, and may make the choice a bit more relevant.