Topic: Washington's War
This is a reasonable rationale, but I would also add that a community of like political minds needs to be supported. If you have a large group of PC markers that is unsupported by any combat or political figure, it is at risk of folding if assailed by the enemy. The empty space does represent, albeit very abstractly, a situation still in flux, but once hard lines are drawn that community needs to be linked into the rest of the rebellion (counter-rebellion) or its morale collapses due to being isolated from its political leadership.
The way I was thinking about it is similar to Joel's well stated view, but I would put an additional spin on this concept. The American Revolution is at its core a war of ideas. The PC markers are an abstract way of taking a Revolutionary War gallop poll. A group of PC markers in a colony represents this war of ideas and a low level conflict that is pitting neighbor against neighbor. The side that is dominant in the colony has sufficient residual energy to generate a militia on occasion. The political energy of a group of PC markers is either expanding, stagnant, or contracting. If it is still expanding, as represented by additional populace open to the political war of ideas (as represented by an empty space), it continues to gain energy. Once it can no longer grow because it has run up against a wall of political opposition, the issues begin to change. The group needs to begin to govern and protect its community, which requires the trappings of government. This is where many local revolts falter and collapse. Once the exciting moments of protesting and public debate have passed the issues change. The new revolutionaries now have to become a governing body, which at its core is ensuring domestic tranquility and providing for the common defense (you might recognize that last phrase from somewhere). Failure to do so drives the situation in the other direction and morale collapses and the situation is once again up for grabs as simulated by the fact that isolated markers are removed, not converted.
Looking at it from a game risk-reward perspective, a one PC investment that is isolated is probably not worth the investment to save, but a multi-colony sized grouping starts to represent a major constituency that is clamoring for support and represents a sizable investment in political capital. The value of the investment should draw enemy attention and at this point the investment is at risk unless you invest in actual military forces to support it. For example a 10 PC marker now requires a one card investment to garrison. Failure to do so is a calculated gamble on your part.
Given this is the big picture of the hearts and minds war you have two main tactics to protect your community of political allies. As the Continental Congress or the British Parliament you need to show your constituents that you care about them and support them. This means troops, so sending even one CU solves the entire isolation issue or for the British connecting them to a port and the British navy. Failure to support your 'political capital' as represented by the PC markers puts your people or said another way your level of political support at severe risk.
The other important tactic is the discard for the removal of a PC markers. This is meant to show, as cited in my design notes, the low level combat that was the essence of the war. The ability of a surrounded cluster to put the situation in flux again by attacking adjacent spaces puts the group back into a potential expansion mode that generates another wave of revolutionary energy.
So remember, especially when playing your last card; make sure that the battle or maneuver that you are planning is more valuable than just removing one enemy PC marker and preserving a large concentration of PC markers. Another similar tactic for the Americans is saving the second reinforcement for a late turn play and just stick a 1 CU army in the middle of a key PC concentration.