pcap-merge is as simple and dummy as its name.
It will not do anything smart, it opens each pcap file, one at a time, and appends packet on stdout.
$ pcap-merge *pcap | tcpdump -nr -
Nope, it just crashes when you have a lot of pcap files to merge.
$ mergecap -w myfifo -a $(ls 201*pcap |sort -n)
mergecap: Can't open 20140119052613.pcap: Too many open files