It all depends really. There are usually bug fixes and enahncements to existing plugins, in each update, as well. If those don't apply to you then it may not be of benefit to update. In the 4.2 update there have been these changes, in addition to the Space Echo:
(Precision Limiter) Added selectable meter type for full K-System compatibility
(Precision Multiband) Resolved issue with level-dependent surge during bypass/enable
(TremModEcho) ModDelay Sync no longer grays out when Echo type changed
In 4.1, in addition to the Precision Multi-Band 14-day demo:
DMA enhancements improve non-realtime renders
(Dimension D) Eliminated click when changing mode
(ModDelay) Depth values over 170c are now recognized by text entry
(DM-1/L, RS-1) Right Pan value no longer lost when saved with project
If none of these fixes/enahncements apply to you, then you may be better off staying with 4.0.
I have the Precision Limiter, Mult-Band & Dimension D plus the other fixes to exisitng plugins are of value to me. In addition I am dying to check out the Space Echo, so I'll be updating for sure.