" It's not as if you're going to be able to implement your ideologically pure agenda with your small ideologically pure party. You only get to power through coalitions. "
99% of the time, I agree. On the flip side, the NDP in Canada have never held power. However, when Canadians list the things they like the most about Canada, they list things that the NDP initially proposed. Other parties stole their ideas to steal their votes, and put them into place. It's not common, but it happens.
One way to raise issues is to be an activist, and one way to be an activist is to start a party, or hijack a party that has similar beliefs. You do it to raise issues that you think are important and hope other people feel the same way, and hope that the idea gets stolen and implemented.
If, on the other hand, you have to sell your soul to a larger party to get power, like the one-party system that Putin33 was describing, you have to give up what you believe in too, which means that people with ambition of holding power move up the ranks, not people with ideas. Stagnation has to be fought in one-party systems.