Replacement for Macro lens?

Recently I've decided to get a few more primes.
Depending on how things work out, I'll have a 35mm, 85mm and 135mm.
Now I've owned my 100mm Macro for 2 years and it comes out very seldom.
My thought was to sell the 100mm, and buy a set of extension tubes to use on one of my other primes.

My real question is; is there anyone out there that has done this? What lens did you use it on? Do you regret it? My issue is, with the 5D, I find that I need perfect edges, and though the Macro isn't perfect, it is better then my other lenses.

Any help on this matter would be great, thanks.

Nothing, no one has done this before?

I use tubes on my 70-200IS and my 85mm all the time with decent results. I am always eying the 100mm and the Tamron 90 Macro for a true macro lens, plus based on my experience the 100 macro makes for a solid portrait lens, but if you want sharpness through, you will have to reduce aperture greatly and have an off camera flash.

My only regret is buying the 28mm 1.8 macro vs. the 100 or 90.

I use a 500D closeup lens on my 70-200 and works quite well for it has two pieces of glass in it, minimizing any negative IQ effects. Closeup tubes won't work as well on a longer focal length as a closeup lens will. With my 1.4x extender and the lens, it gets a lot of magnification. For a shorter prime, tubes will be your best bet.

Since you rarely use your macro, you probably shoot more closeups than true macro shots. The 85 f/1.8 with extension tubes works quite well for closeups as does the 135, even without tubes. Here are a couple sample shots.

This shot was taken with a 30D, 85 f/1.8 and a closeup tube. No cropping was required.

I've used the kenko extension tubes on the 50 1.8, 85 1.8, 135 f2 and 300 f4. Although not true macro, I've gotten some great shots. I will have to post some. If you decide the sell the 100 2.8 put a link to this thread.

Here are 2 photos of the American Tree Frog where I used the kenko extension tubes (all 3 tubes I believe) with the 300 f4. I had to shoot down between a boardwalk railing so it was a challenge to get the shot and not block the flash output, but I persevered. I hand held the shots. The DOF is not the best but that's the best I could do with no tripod (exif intact).

Gear used:
5D2
300 f4 IS
Kenko extension tubes
580 exII with softbox