Although I don't have a HERMS, I've been thinking about making a HERMS some day soon. I was leaning towards it unmounted. I don't like the idea of committing the kettle to a HLT+HERMS. I think that would paint you into a corner if you ever needed to use that kettle as a second kettle. For instance, this week I decided I wanted to make an IPA & a PA. After the sparge, I almost I had enough for a single 10 gallon batch. I split this preboil so I had enough for 5 gallons of IPA and about 4 gallons PA. I topped off the PA pre-boil so I'd end up with a weaker OG PA. This is similar to a partigyle or SWIG where you'd need that HLT as a 2nd kettle. When done, I had two separate beers with only about an extra hour of time.
The path I'll be taking on the HERMS will not dedicate a kettle as a HLT from a flexibility point of view.