Well, it's a lot better than before, where Oak + Regulus = frozen in place. And since practically everybody has some kind of boost to base speed, mark is still probably useful to increase.
Example Base speed = 10.0
Cap = 6.6
Max Movespeed that with a 50% mark would be reduced to the cap = 13.2
They'd need more than a 32% movespeed boost (assuming movespeed still stacks additively) to have a 50% mark not cap the slow. Assuming you have one level of maim (you always have at least one level, right?) they'd need more than 39% increase.
Even with boots and anklet that's only 25%, and few heroes can self boost by 15% more (Sedna being the only one I think, perhaps Oak temporarily?).
So a 50% slow mark is basically all you ever need to get anyone to the cap.