Some Microsoft products provide a dedicated proxy for external access, I'm thinking here about TFS Proxy server or Exchange Edge server.
Why would one prefer using Forefront TMG as a proxy for these services instead of their dedicated proxies ?
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You know, this would be funny if it would not show how little you understand from what you say.
TFS proxy is not for external access, it is a proxy an external location deploys to have a local cache of the data to improove speed. This basically relies on a defined external location (such as a development office) and not an internet scenario (where people come from all around the place). TFS proxy is not a specifc proxy for a very specific scenario. It makes this quite good, btw. - for those scenarios. Making a get on a local repository over a bad or overlaoded line can be brutal, and a local repository cache can really speed things up.
Ah, you mean this serever component that was removed fom Exchange Server 2013 because it was rarely used and MS realized it is a surplus most people don't need?
Sorry, you have a basic extample of someone trying to make a point and choosing ridiculous examples that just are counter productive to his argument. Your two examples are non-issues, one even does not do what you think it does.
That answers the question why someone woupld prefer TMG (which is a retired producto n top - no futher development). What is lefvt is that TMG did a LOT more than proxy. Proxy is / was alawys a side functionality. These days the TMG line lives on in UAG - As access gateway, to have one place where extenal parties access the intranet. Which can be a good thing to control centrally.