Firstly here in the UK we tend not to trust timing belts beyond about 40K miles - Vauxhall (aka GM - UK) did and had loads of problems. As they sold to companies mostly they were spanked by those clients deciding to claim back the cost and/or not buy them again.
We like chains now here
Secondly I think the idea of replacing other items affected by the belt is that doing this means you (should) not have to replace the belt again. e.g. if you replace your belt and then the water pump goes you have to replace water pump AND the belt again. So replacing a secondary pulley, and a water pump and maybe something else makes sense whilst everything is apart.
It does mean a lot of spend but if you plan to keep it then the investment is returned in having newer crucial components IMHO but I understand about the additional spend.