Magic item shops don't ruin the spirit of D&D, as such, but making them as standard as the rules kinda sorta imply really strains an awful lot of settings. But I think the whole item-christmas-tree issue only really starts to break credibility at the higher levels, where a lot of other things get twisted; having as many high-level characters wandering about as the rules kinda sorta imply also really strains an awful lot of possible settings.
(Personally, the rules behind making magic items don't make any sense to me in a fantasy setting. They sort of make sense in a far-future setting, where credits can be swiftly funneled into appropriate resources, and advanced technology allows fabricating complex, customized equipment faster than we can today, thus: powerful enhancements, in a few days' time. As written, the rules allow a caster to make many magic items, individually craft them, in just two or three or four days! It takes longer to lay out a newsletter. It would sort of make sense if it took a month, or a week, perhaps, to make & enchant the magic gloves, and you had to get the right ingredients to do it. Or, it would make sense if you just sacrificed the gold and part of your heroic soul (XP) and poof! gloves of dexterity. But as is, items made quickly out of completely fungible resources, it makes no sense except as a matter of convenience.)