Obviously you can parallel cells up, and everybody does it. But you then have to keep them that way and charge and discharge them together.
I'm a little less happy with connecting strings of cells in parallel, because of the possibility of an imbalance on one string, or of a cell failing or going negative at deep discharge.
If you connect two or more packs in parallel with isolating schottkies, you avoid these problems. The packs can be treated independently, they can be charged separately and used at different states of charge. You can even mix packs of different chemistries and cell sizes. The advantage over running one pack down and then manually switching is that when the packs are paralleled up, they will share the load. This reduces the peak current seen by an individual pack - which can be important for battery life.
I have a 4 into 1 Schottky combiner on my bike, so I can plug in a set of battery packs. The initial reason for doing it was to cut the peak current per pack. I had a Li-Ion main pack with some small NiMH packs to act as boosters. It made a huge difference when hill climbing as the NiMH would add the extra current for the short term when the volts on the main battery sagged.
Nick