No doubt a great glass but here are my thoughts:
If you want to make really good macro shots (especially of living creatures), you only have two options:
a) use flash (no IS needed)
b) use tripod (no IS needed, you even have to turn it off)
So, paradoxically, the pro L version is more suited for the need of an amateur/hobbyist who can take better pictures with it due to IS but the pro should not care (I am implicitly talking about comparison with 100mm macro non-L). The remaining advantages are better image quality (no difference in real use, in my opinion) and weather sealing (I think 100 mm non-L can take some beating too, and who of us shoots in heavy rainfalls or dust storms anyway) which are likely not worth twice the price.
Of course, IS is helpful in "non-standard" use such as portraiture or candid shots but these are not what the glass is primarily made for.