[LARTC] two NICs on the same subnet

[PoWah Wong]

What are the reasons that two NICs on the same
computer are set to the same subnet?
i.e. eth0 IP addresses is x.y.z.m and eth1 is x.y.z.n.
Any websites describing these in details?

http://lartc.org/lartc.html#LARTC.RPDB.MULTIPLE-LINKS
"4.2. Routing for multiple uplinks/providers"
have two cases (Split access and Load balancing) for
two or more internet connections on the same computer
but do not state explicitly that they can be set to
the same subnet.

The two cases requires creating two routing tables,
one for each interface, and route the packets
accordingly (using iproute2), don't they?
These kernel options for either 2.4.x or 2.6.y should
be enabled:
CONFIG_IP_ADVANCED_ROUTER (Networking/IP: Advanced
Router) and
CONFIG_IP_MULTIPLE_TABLES (Networking/IP: policy
routing)
CONFIG_IP_ROUTE_MULTIPATH (NetworkingIP: equal cost
multipath)

Is setting different service (eg. ssh, ftp) for
different interface which are on the same subnet a
valid reason?