I find there's a difference between a reward for logging in, and a punishment for not logging in. Examples:
In Neverwinter (a F2P fantasy D&D MMO) you would get a fancy coin when you log in each day and pray at a shrine. If you collect 7 of these coins, you can get something. If you don't log in and pray within 24 hours (or something like that), all of your coins vanish. If you had any interest, you had to log in every day, almost on schedule, to keep your coins from vanishing. It's not just that you wouldn't gain an extra coin, but the coins you had would be taken from you.
In Planetside 2 (a F2P Scifi FPS MMO) you accrue certification points (this is what experience turns into, and with which you buy new abilities, guns, etc.) when you are off-line. After ~24 hours, you will accrue ~47 certs, but time past that doesn't accrue more. So if you log on every day, you get your 47 certs, but if you skip a day, you still only get 47 certs because they don't accrue past the cap. This is a reward for logging in. You benefit the most by logging in every day, but you don't get punished if you miss a day.
Both of those examples have fairly minor benefits. You can gain 50 certs anywhere from 20min-1hr of playtime (depending on skill and what you do) and I don't remember what the NW coins did, but it wasn't game-defining or anything. And yet, losing my 5 coins made me quit NW and never look back, but the 47 certs has had me logging into PS2 damn near every day for about a year now.
I like carrots, I don't like sticks. Make me feel that I want to log on, don't make me feel like I have to log on. If I have to log in to prevent my coins from vanishing, I'll resent it. I might do it for a while, but I'll be grumpy, and not feeling good about it, and as soon as I've got it done, I'll probably go do something else. But if I don't feel like logging in and all that happens is I go over my cert cap a little, no big deal, it's just an encouragement. And as long as I'm logging in anyway, I might as well go shoot some space people a little even if I only have 5-10 min, and hey, not I'm having fun playing and I might keep playing for a while if I've got time and hey, you've got player retention and investment.
This isn't completely on topic, but exclusive incentives can be seen as punishments, too. A while back (before or around the time it went F2P) I played Rift, a fantasy MMO. They would have events, and by participating in the event you could get event-specific currency to buy event-specific and exclusive items (pets, mounts, etc.). But the event only runs for a limited time, so if you weren't there for the 2013 winter dragon invasion, you can't ever get the special white dragon egg crown (or whatever). That makes you feel like you have to log in and play or else you'll miss out. This is even worse for a F2P game, where a lot of the draw is being able to play casually, and maybe take a couple months off without having a subscription cost draining away.
Don't make me grudgingly log in to prevent something bad.
Make me want to log in to get something good.