Well, two or three things, Uda.
1: I'd be using it on a crop body, and I find the TS-E 24 just too darn difficult to use effectively on a crop body. To really be able to see what I'm doing, I need to use the 1D III, and I don't like doing that - the Mark III lives on the 500 for bird work and I don't like not having it ready for any eventuality - you never know when something interesting will turn up unexpected. Plus it's a right pain to swap over.
Unlike the newer Canons (40D, 50D) the Mark III doesn't have user modes that let you swap instantly from one setup to another. On a 40D, for example, one click of the dial takes me from standard setup (say ISO 200, f/5.6, single shot, single shot focus, partial metering, no MLU) to landscape setup (ISO 100, f/11, single shot focus, single shot, mirror lock-up, evaluative metering, 2 second timer). A second click takes me to bird mode (ISO 500, f/4, high speed repeat, AI servo focus, no MLU, partial metering).
But the Mark III doesn't have that, just a cumbersome save-settings-to-card routine that I never use because it's too clunky. (What a difference a few months make!) So I have to re-set all that stuff every time I want to use the TS-E, and fit the tripod quick-release foot (yes, damnit, I still have two completely different tripod head QR systems - I'll swap the other one over to an Arca-Swiss the same as the big tripod one day) ... and all of that because, not having Lunar's skills, I can't get reliable results with the TS-E 24 using a crop viewfinder - I just can't see what I'm doing clearly enough. Why would a TS-E 90 be any different?
2: I prefer having auto-focus. Even though I use manual focus a lot for macro (mostly using a 40D) and get good results, I use AF for macro quite a lot too. And I certainly use AF for non-macro shots - I don' just use the 60mm for macro stuff.
3: You need close-up rings to use a TS-E 90 for macro work. Close-up rings are a pain, especially when you are swapping back and forwards between close shots and distant shots - something you do quite a lot with flora - you know, entire tree, small branch, leaves, flowers, fruit, buds, bark, tree-in-landscape: it's all part of it. So I don't want to use rings all the time.
4: I'd like something longer than the 25-105 but shorter than the 500. Yes, I have a 100-400, but I don't like the picture quality below about 200mm. It's sharp enough, but the colour and contrast is prety ordinary. At 100mm, my 24-105 kills it. So I can see some benefit in having a nice longish prime in the 130 - 180mm range. (Most people would get a 70-200, but I only shoot in this range a little bit, so I have no plans for one.)
5: I'm not sure that 90mm would be long enough. 180mm might be a bit much (not sure) but I want something quite a lot longer than the 60mm macro I have.
So I might get a TS-E 90 one day - I've seen some superb macro work done with them - but not at this stage.