score:3
Yes, you need an eTA, the rules are pretty clear on the CIC (emphasis mine). So you shouldn't have a problem flying in without one in June, provided everything else is ok
However, from March 15, 2016 until fall 2016, travellers who do not have an eTA can board their flight, as long as they have appropriate travel documents, such as a valid passport. During this time, border services officers can let travellers arriving without an eTA into the country, as long as they meet the other requirements to enter Canada
And as if to answer your specific question, once again the answer is yes, you need an eTA even if there's a leniency period
Do I really need an eTA to fly to or transit through Canada since there is a leniency period?
Yes, as of March 15, 2016, visa-exempt foreign nationals are expected to have an eTA to fly to or transit through Canada. [...] Before you travel to Canada — even if it is when you check in for your flight — you can apply for an eTA using any device with an internet connection, including a mobile phone. It takes just a few minutes to complete the form. All you need to apply is a passport, a credit card, and an email address.
With regards to your last statement, the leniency period is intended to avoid confusion, people have a hard time following rules as it is, this just gives them time to be familiar with the new ones. Imagine you hadn't heard that you need an eTA and upon arrival you're refused entry because the rule allows no exceptions !