idk about infinite, you do lose a bit of time going to get it, but yes, it can work quite well, i like to run the new smite stuff on Morgana mid. Get the AoE smite with the extra health and mana heal, she has spellvamp. Adds a LOT of utility to her, and 80AP and 20% cdr are NOTHING to laugh at on their own.
you can smite the flimsy ranged minions they are hiding behind from your Qs to get free shots at them
you can clear their wards by smiting raptors and finding their ward
red for health, blue for mana smites, Help your jungler clear his objectives faster (particularly that first blue or red since jungle much beefier now) earlier dragons, its a VERY good choice. Or if you dont like the AoE, get the one that slows enemy champs, use that first to garentee a root then follow up with ult
really only works for mid lane, the others are too far away to make use of it, sometimes i run it on support morg for same reasons other than sustain.
who it works on is very limited, but those who do can benefit GREATLY