"I agree completely...
the idea of PM enabled games, is that there are loose alliances, and with that, everything that is involved in politics.. backstabbing when they are powerful/vulnerable, being included.."

Well, that's exactly it, isn't it?

Make alliances into an unbreakable rule, and all the strategy is sucked out of the situation. You declare war on someone weaker than you when you think you can afford it politically, and then you just play it out to a natural conclusion. (You can pretty much always deploy your full income to crush your enemy. And when someone is being "removed" from play, everyone else has no disincentive to participate: might as well jump in, the water's nice!)

The strategy in a Diplomacy game should, rather, be all about balancing your need for defense with your need for cooperation. Just how many armies will you need along your border with your ally? Deploy too many and you risk angering her/him (plus, they will likely try to match the escalating arms race). Deploy too few and you leave yourself exposed, or, worse, actually *entice* them into attacking you.

You must judge the board and try to guess the moment where it becomes advantageous enough for your "ally" to attack you, and play accordingly. That's the real game.

Making alliances a cut-and-dry deal, or building it into the system, is what makes these game stale and boring and essentially un-strategic.