Getting in to SL will bring out the woodworks fans, a much needed boost plus with the bigger teams and more away fans a better atmosphere, all this is common knowledge.
You might lose some fans who have Widnes as a 2nd team who go to games and want you to win but only because their own team isn't in the same league so it doesn't affect them to follow you, but all that said i don't think Widnes' crowd will raise because fans are turning from Saints/Wire just that more will come out to watch the bigger more exciting games.
You may also get the extra boost of Wire/Saints fans coming to watch you if their team is playing on a different day and they want an extra SL fix.
All in all no disadvantage to anyone really just positives. Now hurry up and get here we want our easy 4 or 6(magic week) points a season