This is essentially allowing multis for creation/crafting type guilds.

The whole point of the hiatus system was to allow players a chance to do something else when they needed a break from their primary character.

While I understand the intent of this change is to allow players a way to more easily access products which currently are unavailable due to the low player base, the likely negative side effects outweigh the benefits:
1) A loss of interaction and bartering. When one or two players can completely supply their own needs, there is little need to negotiate with others.
2) A reduction in full time crafters. Why play a tinker/chanter when everyone has an alt to switch to make their own things? This will harm newbies the most, since they are the ones most in need of the "usuals" being around.
3) Devaluation of crafting labor. Granted, the cms system combined with the ability to craft and fight simultaneously has already made most crafting essentially free. But this is just the final nail in the coffin.
4) Favors those with greater play time excessively. At least before you could play a guild with few players and be useful. Now, those guys who can mud 150 hours a week will have an alt in your guild that will render you pointless.
5) Removes an incentive to convince a friend to play. No need to convince a friend to come play a chanter when you can just make one as an alt.

In addition, I question the validity of your justification for this change. You make it sound as though the essential equipment for play is being monopolized and is hard to acquire. But in reality, tinkers are the most common guild and convincing them to make things isn't -that- challenging, especially if you're actually willing to pay for it. Enchanters, while certainly an added bonus, are not essential for survival, and there's a decent number of them too. And remember, you already have the option to hiatus to a different guild if there's something you just can't live without - the restriction of you having to play them for a while only helps the market if there's truly a shortage of that good.