As I read this thread I never got the impression that MLA was being critical....just informative.
Here are the primary issues with speakers atop the dash:
First, they have a significant positional advantage over other or rear cockpit speakers. This can easily be remedied via dedicated amplifier channels so you can bring them into balance. This can also be accomplished via several passive means if the dash speakers are sharing a channel with another speaker. The positional advantage is certainly not a negative and is easily dealt with.
Second, the shape of the dash along with the shape of the windshield creates a wedge that causes a non-linear output.....meaning the speaker's normally smooth response becomes somewhat eratic. Plus, the reflection off glass compounds the issue and makes the speakers sound even more peaky. The phasing issues are not related to other speakers but rather related to the conflicting direct radiation and reflected radiation of the dash speakers being more or less out-of-phase at various frequencies. And this largely contributes to the eratic response. This is a little harder to fix. A sophisticated component set or integral component coaxial with an outboard passive crossover (like a JL Audio) is ideal for this application. You can accentuate or attenuate the tweeter and raise or lower the midbass lowpass filter. With a little experimentation of the settings you can nullify some of the dash location issues and get a much smoother response.
Is mounting atop the dash ideal in every sense? Nope. But you have got to take what the individual boat gives you and make the best of it. And above all else, you need to hear the speakers well when underway and the dash location is excellent in that aspect.
Some boats present audio compromises and every location may present a unique compromise. The key is to choose the leper with the most fingers.