CFL's are not as expensive as they used to be, but you are right about the 4' being cheaper and (perhaps- and I'll explain) more efficient. I think the main advantage of using CFL's is the ability to create more flexible setups and controlling light spread in better ways. for some cases this may also mean using your light in a more efficient way.

Here is a link that might be useful: How to get better prices on grow lights.

Yeah I guess there are some situations where they could be advantageous, but unless you absolutely need them for something it really seems like they'd cost a lot more to use than 4' tubes. You get about 59 lumen/watt with those 105 watt bulbs you linked too. I can get a complete t12 fixture with 75 lumen/watt 6500k daylight tubes for $20 at the local farm supply store. Although the spotlight effect you could get with a cfl and a good reflector could be nice.

The big advantage to the 23w CFL's is that you can buy them practically everywhere, and they're small enough to fit anywhere. Your big T12, T8, etc fixtures are ideal for top-down lighting if you've got the room for it, but for supplemental side lighting or cramped grow spots the "twisty bulb" CFL is king.

Fluorescents don't penetrate a canopy very well (compared to sunlight or HIDs) so the side-lighting can be important for dense foliage.

For me it was a simple question of space. I don't really have room for a 4' fixture nor do I need that big of a light. I only have about 40-50 seedlings. A 4' light would be over kill. I made a small fixture that holds 2 CFL's. Problem solved.

I started using them because of my grow chamber and benc-h. I would have had to get six shop lights, each using 80 watts, to cover the area I have. But three 105-watt lights do the same.

Last year, I found that I could cover ~12 sf. with one 105 watt bulb. Couldn't come close to that with a shop light.

YMMV,

Mike