A lot of tax decisions have long-term implications and similarly some tax laws are written to encourage certain long-term decisions (the mortgage interest deduction comes to mind, for example).

Even though many tax provisions stick around for a long time, there is always the risk that they may be eliminated or modified. My question, does that risk of change affect the effectiveness of Congress' social and economic intent(s) when enacting tax laws? If so, to what extent does it diminish the intended influence of a tax benefit or cost? Finally, what should a taxpayer do in a situation where there is a degree of uncertainty?