We already have two pro-independence parties in parliament who could build towards a place of substance.
VT's point about moving to a position where you have a main opposition who also support independence changing the nature of the debate is a good one, but the idea that Alba are going to be the party to provide a platform for that constructive evolution is fanciful.
Yes, every party has to start somewhere and the SNP were fringe cranks once as well, but considering that the main thrust of people talking up a vote for Alba is that the SNP aren't doing enough to get independence and they should be declaring UDI based on every council by-election they win, these aren't people with an interest in playing the long game and waiting for Alba to become a credible party. These are voters who want independence yesterday, and the internecine warfare in the independence movement which would be unleashed by Alba getting into parliament would put independence further away, not closer; you don't want to wait the years it would take for them to turn into a credible party and stop turning undecided voters towards the union via Salmond's unpopularity.
If you're viewing a list vote for the SNP as a waste and want to maximise the number of pro-independence MSPs, you've got a choice between:
1. A pro-independence party which already has parliamentary representation and a small but established voter base which isn't disappearing, have a full set of policies on various issues, a track record of extracting concessions from the SNP and long established support for independence
2. A pro-independence party who were set up five minutes ago, have no representation or established voter base, no policies beyond 'we want independence faster' but no plan for how they achieve that, a leader whose name is irrevocably associated with sleaze (to put it mildly) and is widely despised across several demographics in every part of the country, whose presence in parliament will turn undecided voters against independence. A common factor among the candidates they've assembled is hatred of Nicola Sturgeon and a desire to remove her as First Minister - you could argue that this is a bigger priority than independence for some of them - and there is absolutely zero chance of this party working constructively alongside the SNP towards independence in the lifetime of this parliament.
If people have reasons they don't want to go for Option 1 due to disagreements with the Greens over non-independence related policies or other issues with them then fine, but anyone telling themselves Option 2 is a better route to independence is absolutely kidding themselves. It is not going to bring independence closer and every single one of you know this.