"I can do all that by getting the source myself directly from the author."
Most people won't, making it a net loss to remove an automated system. Also I'm betting you're not getting the source from the author unless you know the author in meatspace. You're trusting his DVCS (github?) not to be owned and his account not to be owned, then trusting someones gzip / tar program, then trusting their webhost who holds that source code file.
There is the interesting aspect that you probably don't spend all your time on software XYZ, but the package maintainer probably does, so if there is funny business, a distro package maintainer is much more likely to notice than yourself.
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"I can do all that by getting the source myself directly from the author."
Most people won't, making it a net loss to remove an automated system. Also I'm betting you're not getting the source from the author unless you know the author in meatspace. You're trusting his DVCS (github?) not to be owned and his account not to be owned, then trusting someones gzip / tar program, then trusting their webhost who holds that source code file.
There is the interesting aspect that you probably don't spend all your time on software XYZ, but the package maintainer probably does, so if there is funny business, a distro package maintainer is much more likely to notice than yourself.