What am I losing when using extension tubes instead of a macro lens?

After playing around with macro photography on-the-cheap (read: reversed lens, rev. lens mounted on a straight lens, passive extension tubes), I would like to get further with this. The problems with the techniques I used is that focus is manual and aperture control is problematic at best. This limited my setup to still subjects (read: dead insects) Now, as spring is approaching, I want to be able to shoot live insects. I believe that for this, autofocus and settable aperture will be of great help.

So, one obvious but expensive option is a macro lens (say, EF 100mm Macro) However, I am not really interested in yet another prime lens. An alternative is the electrical extension tubes.

Except for maximum focusing distance, what am I losing when using tubes (coupled with a fine lens, say EF70-200/2.8) instead of a macro lens?

Answer

as others have noted you lose light but with extension tubes you gain versatility that is sometimes not available with a EF 100.

I have a EF70-200/2.8 that I use with extension tubes (kenko 12mm, 20mm, 36mm) especially for taking photos of insects. the Minimum Working Distance from the front of the lens to the subject with the lens zoomed to 200mm:

58cm with 12mm extension tube
72cm with 20mm extension tube
86cm with 36mm extension tube

setting the lens to the 70mm position brings your MWD numbers down to the 30cm range

I have the EF 100 macro that I prefer using whenever its expedient but very often the bugs get nervous and skitter away when I get close enough to fill the frame with them. So I use the 70-200 with extension tubes so I can position the lens a couple feet from them and still get very sharp pictures of the bug.

I prefer to not use a flash to take pictures of bugs for a number of reasons consequently another problem with the EF 100 is that when I come in close I often end up blocking the light source or casting a shadow over the subject that changes the dynamic of the picture. By using the extension tubes and the EF 70-200 I can take advantage of a natural light source.

When I do use a flash I use a Canon MR-14EX Macro Ring Lite (I tried a lot of other options and finally bit the bullet) mounted on the EF 100.

as a side note, while purists will sniff disdain, I do stage shots with live bugs - usually what I do is capture several of them the day before and store then in Tupperware in the refrigerator overnight then bring them to the scene I want for a backdrop in a cooler. The cold doesn't kill them but it does slow them down quite a bit so you can stage the shot. Also butterflys are suckers for hummingbird feeder juice made with vodka - they drink some of it then they perch somewhere nearby and move their wings very slowly and don't seem to mind if you get real close to them. Of course if they drink too much and fall over on their side its not a pretty picture.