Nothing will break, but my experience is that having to spend XP in bursts of downtime rather than drip-feeding it as soon as it becomes available is better for pacing and makes for a more satisfying power curve. It also forces players to make some interesting choices in what to train, rather than dogmatically sticking to a pre-determined advancement path they wrote up at the campaign's start.

(Also, I just inherently enjoy fiddling with it.)...