If I may jump in here I agree with both. Since Ruin II is an instant cast that gives you a 2 second window to use an off GCD skill like rouse/spur so essentially you can cast them one at a time without wasting any GCD. This is the same reason why you use fester/aetherflow after using Bio as the 3rd dot. The most important thing is every 2.5 seconds you want to be using an on GCD skill. Summoners can really put out some DPS because while the dots are ticking every 3 seconds for 120-150 Potency or whatever you also add your pet's auto attack + your ruin/ruin II and you are easily doing 200+ potency every 2.5~3 seconds. The major DPS comes from adding all of these small numbers up and consistently using that GCD and not wasting even 1 second on it.

Amelia is not saying to just spam Ruin II she is using it to avoid wasting GCD and keep consistently higher DPS than she would have without it and I agree with that. Now I would not do this in PvP because you would be losing that nice blind but in PvE the blind does not really matter as much I don't think. Most tanks will not rely on something gimmicky like blind as a relevant strategy.

The trick she is using with Ruin II is spread accross all jobs and everyone should realize this and use that because the most valuable resource in each fight we have is time and we should always prioritize the GCD and making sure that every 2.5 seconds a GCD skill is going out.