Most routes of today are very solid with very few loose holds. Where there are loose holds, it is best to not to use them unless you really have to.
However, there are many more adventurous playgrounds where climbing on loose and fragile rock is all part the day out. It adds spice and summons careful skill and judgement. To many, Gogarth (North Wales) has an audacious reputation with more than its fair share of loose and fragile rock, with routes like 'Come to Mother' on red walls (which fell down soon after its first ascent) and Crispin Waddy's 'Death Trap Direct' that comes in at hefty E6 5c, to name just a few.