Your question says "current source" but I think you're really asking about switching voltage to a load.
With a emitter-follower you're stuck with around 600mV of voltage drop (plus whatever the driving circuit drops when supplying the base current) unless you have a higher voltage source available. So if you are driving an LED and have a 3.3V power supply and a GPIO that has 2.9V on it when driving base current, you're going to get about 2.3V to your LED, which will be rather limiting, plus the transistor dissipates power because of the current and voltage lost.
If you use a saturated transistor (PNP or NPN) then your voltage drop across the transistor might go down to a few tens of millivolts, and almost the full supply voltage is available for the load, and the transistor stays cool.
On the other hand, there are circumstances favorable to the use of an emitter follower, for example if you wish to draw current from a higher voltage (perhaps unregulated) supply and provide a relatively constant voltage (when 'on') to a load from that.
Most of the simple questions here don't require that kind of circuit, but I've certainly used it to benefit in commercial products where saving a bit of money can make a big difference.