I am building my latest bike mainly for the reason matther01 said, could buy 2k bike but will never be 100% your preferred parts so you might spend another £500 on tyres, brakes etc or just spend £2.5k from the start and have exactly what you want.
I have a spreadsheet of what I have spend on parts for my ht build and it scares me (£1.5k and counting 8O ) but no way I could of afforded to drop 1.5k in one go.