I'll try to respond to the three items I highlighted above without being too argumentative...
#1) Actually..No. Where I come from the phrase "rip-off" is very strong language. Usually it is applied only when the case pertains to someone who is intentionally stealing or cheating someone.
I don't know, if the lights are equal and 1 costs $90 and the other costs $400 then I would call that a rip off. However I don't think they are equal. lets look at all the differences
Case, cable, circuitry, charger, reflector,batteries and mounts
take your $400 light, use cheaper plastic parts, a cheap reflector off the shelf cable and connector and don't forget the charger ( looks like a transformer to me). Then add in cheap foreign labor and no name batteries , yeah you could probably get the price down to the $100 price range. does it throw out the same light in lumens? maybe, is it the same quality ? doubtfull.
I guess if you're riding around in familiar territory and don't mind carrying extra lights (just in case) oh yeah , and don't crash (don't think the cheap light is going to be durable)
it could be OK. I would rather not buy and carry 2 or 3 sets of lights (not so cheap if you need several) but to each his own
I'm including a link and some pertinent information on LiOn batteries
" Despite its overall advantages, lithium-ion has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current on most packs are is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.
Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for nickel-metal-hydride if exposed to high ambient temperatures. At the same time, lithium-ion packs are known to have served for five years in some applications.
http://www.batteryuniversity.com/partone-5.htm