I think samtools cat would treat the body and the header differently, whereas cat would just concatenate them, breaking the SAM format.

Using samtools will allow you to use -hwhich will copy the header from a selected bam file (default: first file) into the resulting file (regardless of the number of total files merged). 'Cat'ing a bunch of BAMs together with their headers using normal cat would not be pretty. You can also use lists of input files with the -b option.