Not trying to convince anyone of anything but interested in hearing more about your reasoning:
1. By adding bonds, you are taking on interest rate risk (how much depending on your duration) and possibly credit risk (depending on which bonds you buy) - in effect exchanging one risk for another
True, but Bernanke and others have already telegraphed that interest rates will not rise or if they do, will not rise much. Furthermore, I am shortening durations by using a significant chunk of short-term bond funds.
2. You mention the US and Europe, but not other developed markets or emerging markets or other asset classes for that matter
I do not believe that emerging/developing markets have much upside for the next 6 months as well. I've owned separate emerging markets funds (VWO, EWX, DGS) for a number of years now. EM countries need to sell things to developed countries such as gas, oil, widgets. If developed countries are under austerity measures, developing countries are going to suffer as well.
The only other separate asset class I own are REITs. They have already dropped 5% off their September highs.
3. You mention no "major" upside but not that you are expecting downside?
I cannot predict a downside, but I would hope for one after shifting my asset allocation, so that I can take advantage of a drops and buy low. People have been predicting doom and gloom for quite a while and it hasn't happened. I note that since you posted this and my response today, that reported unemployment levels have dropped under 8% which limits downside.
4. Why six months as a time horizon? It gets you past the US election, the US "fiscal cliff" and possibly some developments in Europe (or not) but I'm not sure what else?
My crystal ball gets really fuzzier and bunnier if I write about more than 6 months time-frame.
5. there is also the much debated "missing the best days" arguement:
Missing Best & Worst Days of S&P500 | The Big Picture Note: the more interesting/important points are made in the comments linking to some earlier Mebane Faber articles which suggest that this is not a point to worry about at this stage of the US market
I am very familiar with missing the best days arguments. I am not going to 0% equities, but will still have at least 60% of portfolio invested in equities within a few weeks. I am also more familiar with missing the worst days arguments. I always invest on worst days no matter what.
I wouldn't mind finding some other asset classes that show positive real returns but its hard.