voting is a controversial topic for many anarchists.
some draw the line at voting for local initiatives and representatives (ie voting in areas and on topics that are locally determined, and not voting in state or federal elections). other anarchists vote in any elections, for exactly the point you make, "best alternative to power at the time."
other anarchists never vote, are anti-democracy, anti-representation, and anti-reform, and see voting as conceding to the status quo no matter what form.
if you go to theanarchistlibrary.org and look under topics for anti-voting, there are a number of historical arguments against it. the argument *for* it just seems to be "doing something is better than doing nothing".