If you're abroad for one week per year how much data costs while you are abroad probably isn't a big factor in deciding which deal to get.
That's why I guess it's good to have a choice, but I don't see them as good value because Three so often does better deals on the normal plans.
I suspect a lot is for marketing reasons. A way to upsell the full plans, perhaps even at the same price, so the customer goes away feeling happier.
It's a shame Three introduced them because with the 'make it right' campaign, it has come up with new plans that removed what was previously offered to all (Feel at Home), U-turned on not increasing bills in line with inflation, axed old tariffs and tried to double monthly bills by migrating people from a £15/m plan to £30/m - amongst other things.
(Off topic) A network that is desperately trying to make money (under great pressure from its financial backer) and has a rather unknown future after the O2 buyout wasn't allowed. Throw in a lack of spectrum that's affecting performance in congested areas, and .. well - not really relevant here perhaps.
Ultimately, I think a lot of people are now getting inclusive data when roaming both here and on networks abroad. So I still think that in most cases, you're better off using your own data connection. I realise this doesn't apply to all, but train Wi-Fi has always presented me with issues and thoughts of 'why did I bother?' when I invariably end up disconnecting and going back to 3G/4G.