My answer is simple:

(1) If there is an area which is too dull or requires highlighting LEDs are the shot.
For example - Theatre of Magic's twin upper rollover lane inserts are a notoriously dull green. Despite the clever mirror it's hard to see which lane is lit. Green LEDs solved the problem beautifully. Same applied to TOM's EB and Tiger Saw MB inserts. What a difference. I won't put any other LEDs in this machine now.

(2) If you want to reduce current draw or heat generation for any reason e.g. lit toppers with extra lamps pulling more juice, this can be compensated for by using LEDs.

Generally I find fully ledded machines too gaudy (except under flouro light) and as mentioned above, they can be distracting. Judicious use of LEDs can be fantastic, but it's all inthe eye of the beholder I reckon.