What are you on about, I hear you say? Of course they are!
Actually, according to Mark Russinovich - of sysinternals fame - UAC elevations are not security.
Signature checks serve as proof-of-origin for trust decisions (e.g. installing an activeX control) and integrity check, not as any indication that the software is non-malicious, free from exploitable defects, or carrying a malicous data payload.
The only code in general checked for signature validity during loading are ActiveX controls, .NET assemblies and device drivers. OS components are not verified except on demand. UAC elevations are not 'security' and the signature verification performed by the consent prompt is intended primarily to encourage ISVs to sign their code.