I would imagine that a stabbing attack with an acute point againt scale armour could get caught between scales and penetrate the layer of material the scales are attached to, where it would simply glide off lamellar. I do believe that given the lacing lamellar armour is more difficult to maintain, especially after a good beating or wet/bad weather.
An additional and very important asset mail has over plate though is ease of transportation and equipping it. To store mail you need a relatively small box, preferrably with some oiled cloth around it. To store plate... Well, large coffers! Additionally to don mail armour you put on a gambeson, lace it, and put on the equivalent of a heavy and clumsy t-shirt. It takes a couple of seconds, but bam, you are fully armed. To get dressed in plate armour quickly you need a helping hand or two with the dozens of straps and laces, and you won't be ready in a minute or two.
Now imagine an army on the march which is ambushed. Everyone gets into his armour as quickly as possible, or something at least (if you weren't prepared for hostile contact, and not marching in armour, that is). People with mail are ready in under a minute if they are alert, and are ready to fight. People with plate armour need a lot more time and will fight with an incomplete suit most likely.