The fact that Ryanair are only implementing the additional charges for July and August, the two busiest months for summer sun holidays, combined with my opinion that it's passengers going away for a week or two in the sun that actually require the facility of checked luggage, this just seems to me an additional money making scheme.
If it was a longer term strategy, maybe, but to only apply to the peak summer, looks like extra revenue generation. A city break or couple of days away, most passengers can do with 10kg cabin baggage, but to try and pack for a week or two into cabin baggage, I'd reckon could reduce overall enjoyment of the holiday, and add a heck of a lot more stress travelling.
Ryanair often loudly voiced their opinion on the dinosaur legacy carriers and their rip-off fuel surcharges. I see this as a blatant, hypocritical, luggage surcharge; were the extra cost of checking in baggage with Ryanair, does not accurately reflect the true cost of checking in said baggage. The extra money generated from this, will just subsidise more headlining loss-leading low fares for advertising purposes, and to get bums on seats.