The problem I have with the re-use of the theme song is that, to me, it feels kind of lazy since the theme really has absolutely nothing whatsoever - aside from two changed lines - to do with the Samurai theme of the next season, ...
The song also had nothing whatsoever to do with the changing themes of MMPR.
Having a generic (and well known!) theme that can be reused with minor tweaks for future seasons is not a bad thing. The MMPR theme still makes people turn their heads, it was even used in Guitar Heroes, it's smart to use it, it will get Power Rangers Samurai attention.