You can only focus on league or Cup but not both

[redsnoopy]

Reasons are many -- injuries, tiredness, low morale, poor form....limit to how much power packs can help

Not really.
Why would you not be able to focus on all matches? I'm not seeing you mention any reasons at all.

[Killerbee]

Reasons are many -- injuries, tiredness, low morale, poor form....limit to how much power packs can help

1) You can't cure form, so there's no guarantee that benching/swapping a player will lead to him regaining form.

2) Injuries will always happen, but again there's no indication on which players they will affect nor does it matter if you focus on cup/cl/league. If a player is injured, he'll be injured for all competitions, not just one of them.

3) Low morale is no issue. They are the easiest packs to earn (up to 3 per game, also applicable on friendlies) and are needed in very low amounts. (2 packs even heals morale to 100% in case you have a player you need to play who has been benched down to 0 morale)

4) Tiredness is annoying, but not more than that. Over the day you have enough stamina for 2 matches and possibly some trainings. The only issue is the match scheduling in which the games may follow one after another.

For point 3 and 4, it's been widely questioned whether or not condition and morale have any influence on games. If they don't, then those 2 factors become even more irrelevant.

Basically you're saying that the packs you earn in regular conditions are not enough to keep a team in shape for games in multiple competitions. I don't agree with this. With the packs from friends and occasional packs from watching videos you should be fine to compete in all competitions. Only the typical injury frenzies can hurt you, but even that shouldn't stop you from competing.