Why is there a limit on the number of counters allowed on a Hadoop card to reduce performance?

I use Hadoop map-reduce and get a CountersExceededException when I try to programmatically create many job counters. I know that I can increase the number of counters that are allowed through configuration files, but does anyone know:

a) why there is a limit on counters with reduced display

and

b) if it is a good / bad idea to increase the maximum number of counters with decreasing the map?

I am using hasoop 0.20.2