What starbucks_mafia said is more accurate. If you use "item extra sutff" as your item name, mIRC should search for the item named "item extra stuff" and return $null, not search for "item".
The lack of support for spaces in item names is an implementation limitation, not a design limitation. It is theoretically possible for an item name to contain spaces with proper escaping or quoting, it's just not implemented currently. Since there is no standard for ini files, there's no reason why mIRC can't extend support for item names with spaces in a future version. This functionality should be therefore reserved for future use, which means relying on searching just the first word would/could break things in an update.