You can leave them but be aware
Absolutely you can leave them on your projects list, although as Kelly says you need to have at least one next action/calendar/waiting-for. The reason is that if you find yourself in the right context and that next action is available for you to do, you will be in a much better space to choose to do that. (As opposed to having to go look at your projects list and then define a next action that may or may not be appropriate for the context you find yourself in.)
Try to gauge your feeling during the weekly review as you consider each project and think about the following:
1. Do I have to do something about this this week?
2. Do I intend to do something about this this week?
3. Would I like to do something about this this week, if I have time?
For me, a yes to any of the above and it (along with the next action) stays on my list. But I have a lot of personal interests and I do not like to limit my options so I'm comfortable with seeing a lot of projects and a lot of next actions. Some people get overwhelmed if their lists are too long. Everyone has a different threshold - you may do better by selecting only 5 or so personal projects to keep active. So try moving something to someday-maybe and see how it goes.
Don't forget that someday-maybe is not a punishment! Also, just because something is on someday-maybe does not mean that you can't do it should the stars align and opportunity arise. If you wanted to golf every golf course in the city and found yourself being invited to that private course you've never been to, you don't say no because it's on someday-maybe. If buying some new knitting needles is part of a someday-maybe project and your favourite craft store has a super-sale you can still go buy those needles. The trick with this is to review your someday-maybe list often enough to know that you'll recognize these opportunities when they show up.