It is a bit due to the fact that developers may sometime need to "hack" the Application to add certain features.
For example SIMBL is needed so we can enjoy 1Passwd, the password manager in Omniweb, e.g. it is supported.
I think one should question why they need to "hack" the Application rather than blaming them for all problems on system. I tell you the reason as an end user: Apple doesn't provide the API's allowing such functionality in "legit" ways.
Also for the record, SIMBL is open-source and free (donationware)
http://culater.net/software/SIMBL/SIMBL.php
Developer of it makes it clear that it shouldn't be needed:
It seems like this functionality would have been built into Mac OS X already, so I may be duplicating some native functionality - I hope not, as that would be a waste of time.
So, perhaps Apple is acting a bit "secret" on the API so people has to "hack" the system to maintain some needed functionality.
One should disable the "Input Manager" by dragging it to Desktop and test the application crash state. If it does crash, it is not Input Manager's fault. If it doesn't? People like me reports to the Input Manager/Haxie developer before trashing it and wait for new version.