The nice thing about rabies, though, is it allows the mage to think about where they are- the drop off point is pretty quick. The threshold starts at 6, and every meter away from the vine you are decreases it by 1. It's almost the reverse of those spots in SR: Hong Kong that boosted magic.
Unlike background count ("you take -2 to all magic, 'cuz"), the vines are something that players can plan around. This can be tactical, but it can even be on a strategic/planning level ("the safest way to the macguffin brings us through MCT's thaumatogical research lab. That could put mage out of commission. There's a longer route, but we're more likely to trip the alarm if we go that way")
With all things, sure, the GM can use and abuse it so that the entire area is covered with vines. There's nothing stopping them from making all corpsec lesser dragons with panther cannons and make all the hosts have eleventy-five rating, either.