I've been thinking of the same idea and my understanding is that if you charge any batteries in series (eg 2S1P or 2S2P or 2S4P, etc) there in nothing to stop any 2 batteries in a 2S pair from becoming unbalanced. Using 'good' cells reduces the odds of problems since these cells are supposed to be more closely matched. BUT, you are still charging them 'open loop' in the sense that there is no feedback to signal that they are becoming unbalanced. If one battery loses capacity sooner than the other then the pack becomes useless or even dangerous until the batteries are rebalanced or replaced.
Based on this and magnitude of what can go wrong, etc it just seems much more sensible to have a pack that can be balanced charged occasionally, either by having separate leads coming out of the pack (ugh!) or having removable cells. I'm leaning towards trying to gen up a carrier that I can just plug in for most charges and then easily disassemble every so often to check and re-balance the cells.