To be fair I don't see why it's not just made that when you buy an item with +mana pool, it doesn't automatically give you the mana. Since all shops (aside from artifact ones, boohoo) are right beside a regen crystal I don't see it as being much of a problem.
Well the artefact shop is a problem, since you get items there with much greater mana and health increases than in the item shop. I don't think anyone would want to
not get the additional 1600 health filled right up when buying the Girdle of The Giants for example, somewhere in the middle of the map...
Also I don't think anyone wouldn't want to get the instant health bonus when leveling up in mid-fight for example or when using a skill point for Enhanced Attributes. Sure, you could draw a line between items and levels & skills, so that items will not fill up the additional bonus but levels & enhanced attributes will. But that seems very artificial.