How does PS+ work anyway? Do you get around five new free games per month, plus exclusive discounts on some games in the PS store?
If that is the case, you only need to think about how this affects gamers and games. Must have titles won't be negatively impacted by PS+, because these games will still be bought early on. However, everything that falls under the "maybe" category and was planned to be purchased once the price has dropped to around $30 can now be waited for to arrive on PS+ (provided the service offers a good selection of games; if they only put duds on it, it won't really matter). If you previously bought five games per year that fell into this category, you were spending $150; with PS+ you are spending $50 for the same and more. If your gaming time is limited, then PS+ might have an even bigger impact on the money you spend per year.
Aside from the most dedicated PS owners, I can't see anyone whose spending on gaming would go up because of PS+. Rather the opposite is going to be the case.
Things like exclusive PS+ sales, additional money off in a sale, day one discounts for PS+ owners etc I'm sure pushes people into buying things they otherwise may not have.
If they give a game away in PS+ it will not be the GOTY edition or have DLC included also, I'm sure some games are given away on it with the intent to get some DLC sales, on older games that are cheap at retail and selling slow especially (and likely have a lot of used sales happening) the DLC sales may make more money even.
Also quite often when a sequel to a game is coming out in preceeding months the game that released before it finds it way to Plus, obviously used as marketing tool.
It is still a rather long wait before a game goes to PS+. Except when they give a brand new PSN only game away on the day of release...I cannot figure that one out.
On Vita I think it is more pushing people into buying a Vita, as well as buying overpriced Vita memory cards.