they have become important, but it's hard to compare to the 70s and 80s because the 9 masters system is very new relatively speaking. So you can only judge player's greatness based on masters results, with players from maybe the late 80s onwards. People even forget Lendl won 22 equivalent titles.
But masters are more relevant to the last 20 years than anything before. In terms of titles that have been recognised across the history of tennis, and when they acheived approximately their current level of importance we have
Wimbledon
US Open
RG
All recognised at current level throughout the vast history of tennis
WTF/Tennis master cup etc
Recognised at current level since early 70s
AO
Recognised at current level since late 80s/early 90s (probably being generous with this one)
Masters
Recognised at current level since mid 90s probably
Olympics
Recognised at current level in the last 2 to 3 events.
This is open for debate though, and individual masters events have been at their current level of prestige for longer than others, for example Rome, MC, Cinci