Why NAT is a disadvantage when TCP source port field is 16 bits?
The practical limit to the number of hosts that can use a single NAT'd address is much less than 61,440.  This is because even a modest Internet-capable computer will use many simultaneous TCP connections to hosts beyond the firewall.  Each of these connections, while it is open or being negotiated open or closed, counts against that 61,440 figure.  If each local host needs to reliably be able to initiate 200 external TCP connections at once, then that 61,440 value is reduced to only 307 hosts.  This won't be a problem for a typical home network (yet), but a small business could easily reach this limit.
If the TCP source port size were larger, then that would offer a correspondingly larger number of address:port pairs for address translation to use, permitting many more hosts and connections from behind the firewall.