A preheat tube is not really necessary for temps above freezing (32F.) Below that, they become increasingly useful down to about 20F, as Jerry says. Below that they are necessary.
I never camp in winter anymore, and, I never use a canister stove. I tried several but they either broke, started sputtering & flaring. They were always a bit unreliable. So, as with the past twenty or so stove stove purchases, I went back to my SVEA. A bit heavy at 17oz (weighed on a digital lab scale at 16.97oz) it remains competative with any self contained stove on the market, even today.
Anyway, the preheat tube establishes a "known" temperature/pressure to the gas. It doesn't really matter what type of fuel. At a certain flame height, it gets x amount of heat from the flame. Since temp and pressure are related, this means a relativly constant flow of gas to the jet to mix with air, and to the burner head, given minor fluctuations in input pressure. White gas, Kero, etc require a preheat. (The SVEA uses a combined valve/brass heat exchanger to preheat the white gas into a vapour, same principle in a smaller output stove at ~4500BTU. It does not work for larger stoves all that well.) While Isobutane/propane mixes do not "require" a preheat, they run better with one. The windpro for example works very well, running very consistantly down to 20F.
Any fuel MUST mix with air before it will burn. This means that everything (well, 99.9%, explosives are different) must be in a gas form before it will burn...wood, plastic, wax, gasoline, whatever. So, by maintaing a constant heat on the gas, they can guarantee a constant pressure, hence consistant burns.
This solves the consistancy of burning problem, but does NOT solve the step functions associated with canisters. You get the full 11oz or 7oz canister every time. You cannot adjust it. 7oz isn't that much, but is significant.