If a call to the Maxima() function is extended to a second argument, this argument can't be deleted using backspace. in 6179 (stable) this problem doesn't exist. Also, after disabling Custom Glyphs plugin, the second argument can be deleted as usual.

Copy this to a math region and try to delete the empty string and the placeholder:
Maxima(f.x(a,b),"")
The maxima plugin is the current svn state (no local mods).
SMath Studio: SMath Studio Desktop 0.98.6398.26177, portable, x64
OS: Microsoft Windows NT 6.1.7601 Service Pack 1 (Windows 7 Professional Service Pack 1, x64)
Screen: 1920x1080 px, 96x96 dpi
Localization: separator=<semicolon>; decimal symbol=<comma>; system decimal symbol=<comma>; language=ENG
Frameworks: CLR 2.0.50727.8669
installed versions:
v2.0.50727 2.0.50727.5420 SP2
v3.0 3.0.30729.5420 SP2
v3.5 3.5.30729.5420 SP1
v4
Client 4.6.01055
Full 4.6.01055
v4.0
Client 4.0.0.0