ARP flux, why not "disabled" by default?

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Hi - hopefully a good place to discuss this (if not, where else?)

last week encountered something unexpected, basically what was ages ago 
described here:

http://linux-ip.net/html/ether-arp.html#ether-arp-flux

A host (A) with three network links

eth0 in 10.0.0.0/8 network
eth1 in 172.26.0.0/12 network
eth2 in a.b.c.0/24 network (outside, open to the world)

we added a couple of virtual kvm guests and created bridges for each of these 
interfaces (hence moved the Ip to the bridges) and created tap devices where 
necessary.

However, we expected sudden connection drops and sometimes multiple responses 
to pings (and arpings).

Tracing it down, we received ARP replies on the wrong interfaces, i.e. when 
sitting on a machine (M) in network 10.0.0.0/8 with a gateway (G) allowing 
hosts from this network to the external a.b.c.d/24 network/

When creating an arp who-has on the external IP a.b.c.d, host A replied with 
its MAC address participating in network a.b.c.0/24 but on network 10.0.0.0/8 
:(

After a lot of trial and errors, setting

net.ipv4.conf.*.proxy_arp = 1
net.ipv4.conf.*.arp_filter = 1

helped, but left us with a couple of questions:

* Why is this not the default for the linux kernel, I guess there is a very 
sane reason for that, but I have not found it so far

* what do these functions actually do? I'm too bad a programmer to read (and 
possibly) find the code pieces, and chances are very high I won't really get 
it from there. Would anyone be so kind and lend me a hand/link where I can 
learn more about this?

Cheers

Carsten
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