Other than the fact that it will be around in its present composition indefinitely, tell us why you think recycled rubber - essentially petroleum - would make a good growing medium? Zero nutrient content, an undefined array of possibly toxic substances which certainly have the potential for leaching out of the material, no ability to retain or even slow water penetration (just as essential as good drainage).....doesn't sound like a choice material to me. In fact, the only appropriate use I can think of for recycled rubber mulch is as a cushion in a child's playground area, ie., under a swing set, for which it is used rather commonly in this area. Even better than play chips, as it is very resiliant, won't need replacing in this lifetime and no chance of splinters.

Perhaps it is just me, but I find little practical use for inorganic mulches of any variety, with the exception of rocks/gravel for scree gardens. If aeration and good drainage is your primary concern with a container medium, suitable proportions of perlite or pumice would suffice nicely, no doubt for substantially less cost and are at least natural, nontoxic substances.

As a follow-up to this issue, the Jan/Feb 2006 issue of Garden Design magazine has a short article on the pros and cons of rubber mulch. While this publication is not known for its technical articles (tends to be more visual and fluffy), I found this encapsulated info to be particularly relevent to this discussion.

The article was a review of various scientific studies conducted with product and seemed to be rather unbiased. On the positive side, it reports that the primary plus was that it made use of 100% recycled materials. It also noted that when used as a mulch recycled rubber won't attract insects or termites and has a 95% weed supression capacity. On the negative side, it has the potential to leach zinc into soils, it is susceptible to combustion, it can include ground steel (steel-belted radials), which can be dangerous, it has an unpleasant chemical odor at least when fresh and it takes 100+ years to breakdown. The final recommendation was that it is a product that is best used for decorative rather than strictly horticultural purposes.

Probably the most telling aspect of the article was a quote by Rufus Chaney of the USDA Agricultural Research Service that advised that for the zinc factor alone "rubber should never be used in gardens or compost". I'd certainly expand that to include container gardening as well.