It takes 1 instance of work every 6 months for drivers to maintain competence. Carmarthen depot is larger than Holyhead and Junction combined, and Cardiff Mainline is a massive depot, so one trip once a week isn't going to do the trick down there.
That said, there's been talk of Holyhead, Junction and Chester all losing their 150/153 competence for a while now. It wouldn't surprise me if some of the crews at the first two depots are already either relying on refreshes or simply no longer competent. I suspect in the next year we'll reach the point where in the North only Crewe and Shrewsbury sign 153s, and nobody at all signs 150s.