I think it varies from club to club. And clubs will have their own way of justifying why they attack in a certain way. Similarly I find it amusing sometimes when people run across the room doing an attack.
In my mind, as long as the trainees know WHY they are doing an attack in a certain way it doesn't matter. i.e. slow, weak attacks can be useful to enable people to go through the technique step by step, or maybe even to simulate a dummy attack. However training like this consistently is ineffective. There needs to be variation in attack. That's why I think training with different clubs is a must, and also analysing the purpose of the aikido training method.
I think the real reason for weak attacks is in many cases just lack of enthusiam, which then becomes a habit.