Why would one declare a Java interface method as abstract?

According to the Java Language Specification, the abstract keyword for interfaces is obsolete and should no longer be used. (Section 9.1.1.1)

That said, with Java's propensity for backwards compatibility, I really doubt it will ever make a difference whether the abstract keyword is present.

"The benefice of that" (adding abstract on interface methods declaration) in eclipse would be an old compatibility issue with jdt eclipse compiler in jdk1.3

Since 1.4, jdk libraries are no longer containing default abstract methods (on abstract classes implementing interfaces).
This is fooling the Eclipse 1.3 compiler diagnosis since their implementation is relying on their existence.
Note that Javac 1.3 would refuse altogether to perform against 1.4 libraries (using -bootclasspath option).

Since the Eclipse compiler is likely to be in 1.4 compliance level (see Workbench>Preferences>Java>Compiler>JDK Compliance), or use at least 1.3 class libraries if using 1.3 compliance mode, the presence of "abstract" is not required in most of the current eclipse projects.

From the Java SE 7 JLS (Java Language Specification): "It is permitted, but discouraged as a matter of style, to redundantly specify the public and/or abstract modifier for a method declared in an interface."

For Java SE 5.0: "For compatibility with older versions of the Java platform, it is permitted but discouraged, as a matter of style, to redundantly specify the abstract modifier for methods declared in interfaces."