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I find it odd that there actually isn't more clarification on this issue from SI being that most of the people who play the game enjoy the depth.
From what I've found in my experience and searching a zillion posts on here as well:
The main point of having leagues on view-only is to process fixtures and stats. Those leagues will have much less activity from a game play standpoint.
If you don't plan on managing in a country or accessing a ton of players from that country, don't put it on playable. If it's an important league from the standpoint of buying some players, loaning, scouting, etc., then set it on view-only. Other than that, the higher rep players from the other nations will still be there to find.
Currently my save is large database, 30 leagues, 20 nations with 8 as playable (England top 4, Scotland top 2, Holland top 2. The rest Germany 2, Spain 2, Italy 2, Germany 2 and the top division of a bunch of European natipns. Nothing outside of Europe and all Irish players loaded. (I play as an Irish manager)
I know this may be asked:
1.3GHZ, 1GB RAM
I sometimes have to wait up to two minutes to process fixtures on busy days, but I don't mind.