Yes, a faceplate change is a million times better for you and the customer, EXCEPT when they have the hub or similar upstairs on an extension....
Although if they have 6 core running up there already I've never, ever had an issue with making the green pair, for example, DSL only from the unfiltered block on the plate.
I then change the affected socket for a double euro with one BT on the blue/orange pair and one RJ11 for the modem only on the green etc...
To be honest not had to do it for ages though with WW and 3G type comms.
Yes, I think it's intentional.
As you say most will just see an increase and be happy, even if it could be even better, they'll never know.
In dire cases they'll phone BT but if it saves 3 out of 4 home visits they're laughing.
Here we go, it's official, Infinity self install, even mentions 'if you DO already have a 'double socket', 'otherwise use plug in filters...'
http://bt.custhelp.com/app/answers/detail/a_id/47225/~/help-with-setting-up-bt-infinity-yourself