It was kept around because it was how banks counted coinage you deposited. You'd have little plastic money bags (remember those?) with 1p/2p 5p/10p "NO MIXED COIN" written on them, the person at the desk (or Post Office) would just weigh it to save having to count it all out!

I remember my dad explaining to me in the 1980s that ones and twos didn't count as mixed coin after I'd separated them all out into different bags!