If they decide to issue $100B in tax credits or spend an additional $100B on roads, the budgeting result is the same. Refusing to look at tax expenditures in the same light as spending leads to circular "starve the beast" logic, which, while nice for conservatives, is bad for public policy when people don't actually want to gut all of those programs. If you're actually concerned about deficits (nobody really is, it's just cover for other policies), then you should be treating both sides of the ledger the same way.