Just that it would be impossible to separate the outhouse earth from the house earth, therefore there has to be a correctly sized bonding conductor between outhouse and house. As previously said, a water pipe (metal of course) could be used as that bonding conductor but not a gas pipe.
As you say, in the case of a
water pipe, that is 'self-fulfilling', in that every part of the water pipe (including the bit that enters the outhouse) will automatically be 'adequately bonded' (by itself) to the house's MET, and that would remain true even if it didn't "need to be".
In view of that inevitable 'automatic bonding' (whether one wants/needs it or not), the question of whether it "needs to be" (adequately bonded to the house earth) cannot be sensibly discussed. However, although, like you, I cannot recall having seen a gas pipe going from a house to an outhouse, I imagine that it occasionally happens (and is probably 'allowed', provided it was installed by someone 'allowed' so to do), so we
can discuss the matter in relation to such a gas pipe.
On that basis, and reaching for my 'electrical common sense' hat (rather than a 'regs' one), I have to ask
Why it would be necessary to have "a correctly sized bonding conductor between outhouse and house".
As you know, the (only) reason for main ("equipotential") bonding is to turn a space into an equipotential zone, so as to prevent dangerous potentials existing between simultaneously-touchable parts - and, in the case we are talking about, the relevant 'space' is the interior of the outhouse. That can be achieved by bonding together all simultaneously-touchable parts within the outhouse (if, say, it was TTd), without any need for 'bonding' to the house's earth.
We're really talking about the opposite of the normal situation, particularly with a TN-C-S installation. The concern is usually that an extraneous-c-p will introduce 'earth potential' and that this could (under very rare fault conditions) be very different from the TN-C-S 'earth' connected to exposed-c-ps within the property. However, with a TTd outhouse, the greatest concern would probably be that the extraneous-c-p might (under those rare fault conditions) introduce a potential considerably higher than the local (TT) earth to which exposed-c-ps were connected.
An earth electrode at the outhouse would simply be an unnecessary intentionally added extraneous-conductive-part.
As I've said in relation to SB in bathrooms, the most foolproof (and, sometimes, only) way of ensuring that there cannot be dangerous PDs between simultaneously touchable parts is to have local bonding connecting them together (i.e. 'SB'). If, rather than TTing it, one uses (only) an exported TN earth in an outhouse, then bernard would probably point out that the greatest 'risk' (under extremely rare circumstances) is to someone who has 'one foot/hand inside, and one foot/.hand outside' the outbuilding.
In passing, I often wonder about the oft-seen assertion that if one connects an earth electrode to a TN earth, that electrode constitutes an extraneous-c-p. However, I'm reminded of the last clause of the BS7671 definition of extraneous-c-p, which is "
...and not forming part of the electrical installation". Is an earth electrode not 'part of the electrical installation'??
Kind Regards, John