A tile editor doesn't have the same global use as other tools in Construct. Since many people are making non-tiled games, having a tile editor wouldn't be justified 100% of the time, especially considering the amount of work that would go into it.
I agree that a "Tile Blitter" object like the one nobuyuki described is a better idea, but there's just one problem... which tile map format would you choose to work with? As soon as you decide on this, you'd have people screaming at your for that or the other.
Currently in the klik community people who have need of tile editors write their own custom ones for their games. I suppose it would be possible to make a generic one that would work in conjunction with a Tile Blitter object, but it would take a lot of work to get enough features into it to satisfy people using it. Perhaps some enterprising samaritan would be interested in tackling that...?