The advantages of thinning are (a) the plant has to make fewer stones for the same weight of fruit (b) the plums are much easier to prepare if there's a sensible quantity of flesh on each one and (c) the branches are less likely to break, as mentioned above. But yes, do prop them up if there's a chance of them being overladen even after thinning. And (d) if you don't let it work too hard this year it's less likely to take a year off next year.
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The advantages of thinning are (a) the plant has to make fewer stones for the same weight of fruit (b) the plums are much easier to prepare if there's a sensible quantity of flesh on each one and (c) the branches are less likely to break, as mentioned above. But yes, do prop them up if there's a chance of them being overladen even after thinning. And (d) if you don't let it work too hard this year it's less likely to take a year off next year.