Reasons for devs to want to be in IGC:
1) They get paid money. How much varies from dev to dev. But remember, these are mostly very old games that no-one is buying new any more anyway -- so any money is an improvement over the no money they are making today.
2) It's a way for indie games to build word-of-mouth. That can outlast the month the game is free, or spread to other platforms, making indirect income.
3) It increases (perhaps vastly increases) the pool of potential customers for a game's DLC.
4) It can raise awareness of a game series e.g. put a game on PS+ just before the sequel comes out.
Gamersutrs interviewed a number of devs who's games had been in the IGC. They were happy with how it had worked out.
I also thing PS+ isn't as startlingly good value as it appears to be.
Does it seem outrageously generous to you? Fifty-odd free games a year, split across disc-based bigger titles and downloadable smaller titles, for £40 a year? To the consumer that looks like great value, so logically to the industry it must represent a terrible idea, right?
Here's another way to look at it. It's quite well-documented that the attach rate for most of this generation of consoles hovers at around ten or eleven games. That's across eight years they've been on sale. So the average person buys a game every year, plus a couple of extra across the years. It's a much lower figure than we are used to thinking about, because most of us get through that many games a year. Suppose they pay full price for half of those games and buy the other half for £25; that's a total lifetime spend of £325.
Now consider a position where a person buys a console, gets PSN+, and renews it every year and never buys another game. You'd think this means they're a dead loss to the industry. But actually, assuming they've spent £40 a year on PSN+, that's a total lifetime spend of £320. They're not a loss to the industry at all -- they're actually just a regular consumer. All you need to do is sell that person one game every two years and they're suddenly great customers spending well more than the average joe.
So from the industry's perspective, I don't think PSN+ is the massive value sink it's often presented as. It actually creates value.