Generally a tincture is much stronger for most herbs. Also, for many you do not want to drink the herb as a tea. Lobelia, for instance, tastes terrible.
A good example is most of the calming herbs. A few drops of a skullcap tincture quickly relaxes. A full cup of tea is nice but it doesn't pack the same punch.
Now, remember, herbalists are not acting like traditional MDs. More is not always better. Generally if you have the right herbal preps - just getting them in is good.
Also, Dr. Christopher recommended tinctures for his sickest patients. He also used some things in the form of tea because you can obviously get a tea made much much quicker. Both have their benefits.....so have both! (but don't go overboard - there's usually no need)