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I'll second Tiburon; that's just a personal take, though many DMs I've met adopt a similar house rule.

There are a couple of reasons for restricting spells from outside sources:

1.) Setting specific design issues. The Frostburn supplement is designed around an intensely cold setting, where cold resistance and immunity are fairly common, and where the [ cold ] subtype is much more common than in a standard D&D game. One consequence is that spells like shivering touch are balanced in the setting, but become unbalanced when used outside that setting.

2.) Setting style issues. Conversely to point #1, the setting may have genre conventions that get badly disrupted by non-genre spells; Arcana Unearthed has some strong restrictions on coming back from the dead, so importing spells that defy that convention would be unbalancing.

3.) Power Creep. This is a big one. The spells in the PHB are considered "baseline" spells, but as the game progressed, more & more options became available for characters. Aside from a few very conspicuous efforts to reign in spells (3.0 converting to 3.5, a few cases in the Spell Compendium, and so on) newer spells tended to be more powerful than the baseline spells. Not so much more powerful that they were the wrong level, but still often a much better choice than the Core spells.

Requiring research (1 week & 1,000 gp per spell level) is a way to address the issue of #3. Look at Lesser Vigor (Complete Divine, p186, 1st level Cleric spell); it grants Fast Healing 1 for 10 rounds, +1 round/level, to a maximum of 15 rounds, to one living creature touched as a Standard Action. For non-combat healing, this spell is completely superior to Cure Light Wounds. Even for combat healing, it's pretty strong given that an unconscious PC will automatically stabilize and continue healing. The only lost utility is offensive use against undead.

Now, would I ban that spell for being overpowered? No.
Would I object to that spell in a campaign for balance or setting/style reasons? I can't imagine a scenario where I would.
If this spell was in a campaign, would players ever choose a wand of cure light wounds over a wand of vigor? Not very often, which suggests to me this spell has some value above & beyond the core spells. Is 1,000 gold too much to pay to have this spell as an option for the rest of the campaign, including item creation? I know it's a big hit for a 3rd level PC, but obviously, there is some value in this non-Core spell, so it seems appropriate to assign a value to it. Gold is a renewable resource for most PCs...