Well I'm guessing that a lot of them are compatible between 1.5.1 and 1.5.2, such as GregTech.
Soul shards and Mystcraft focus on more obscure mechanics that are more likely to be changed on short notice.
And when it's summer and not raining every day, people tend to have a lot more distractions than in winter.
In winter Inspent like 2-4 hours a day on MC, but lately I have only been on like an hour a day, most of that spent trying to work Get 1.5 working (first time using MultiMc)
But luckily neither of those mods add world gen.
Creating an age and then removing mystcraft would probably be VERY bad, however.