It's the cost involved when using off the shelf disposable batteries. If you're just doing an hour or so a few days a week disposables and a standard light are just fine. If your like me and do hours in the dark you really need rechargeable batteries and brighter lights. I built a hybrid lighting system. I use a dynamo to power a 1-watt LED headlight, 5000 MCD 30mA taillight, and recharge my Ni-MH battery pack. Below is a drawing of the circuit I use.
When my speed is 10 MPH or higher 120mA of excess current is recharging the batteries. This happens to be the suggested slow recharging rate for Ni-MH batteries. Below 8-MPH the batteries keep both head and taillights on. I also have the luxury of being able to disconnect my dynamo when climbing hills and keep my lights operating. I need to do that every now and then to prevent overcharging the batteries. In the event my speed is slower than 8-MPH and my batteries die I can open S1 and rapid recharge the batteries with 500mA, which happens to be the suggested rapid recharge rate. Switch S2 is only used with hub dynamo systems to disengage the dynamo and not needed for bottle or BB type dynamos. My Ni-MH battery pack is rated at 2.4 amp hours and can power my lights for almost 3 hours.