The whole set-up of the Parliament has the Prime Minister as a direct audience. Remember how often Trillanes made his bogus statements against President Duterte? It wouldn’t be much possible under a parliamentary system because there would be grilling from both sides. It doesn’t matter which side he belongs to because he will have to substantiate his claims against the prime minister and explain why he’s siding with Deputy Prime Minister Leni Robredo.
If he can’t substantiate his claims then he can’t use whatever achievements he had or scholarships to defend his possible removal from the parliament. Frequent scrutiny would have probably unveiled whatever secret activities he’s been up to. Also, would have the Philippines been turned from presidential to parliamentary — he may never even been allowed to be part of it in the first place.