In most cases, I would agree with your premise. There are one or two areas of speaker performance, however, that can make a speaker more or less HT-friendly.
First is dynamics. Generally, soundtrack reproduction can have more bombastic dynamics than music. While some music may require similar dynamic abilities of loudspeakers, many listeners are happy listening to music (e.g. acoustic Jazz) that does not present dynamic challenges to loudspeakers.
Second is overall level. While some HT enthusiasts strive for Dolby reference levels, not all music listeners listen at such high SPLs. Again, some loudspeakers may sound great at lower levels, but don't hold up well when pushed hard.
Third is Soundstage and Imaging. While a dedicated HT system will usually rely on a digital processor to place film sounds in their proper places, 2-channel music systems rely on the electronics and the speakers to provide a sense of space and performer location. So, a speaker with poor imaging or soundstage abilities might still work well in a 5.1 or 7.1 HT system.
Fourth is bass extension and output. In addition to dynamics and overall level, a speaker system for a HT requires extended bass capabilities that are both deep and loud. Sound effects will usually place more demands on a HT system than will most music. If the HT system uses a subwoofer for the bottom few octaves of all channels, the mains will do well even if they don't have much deep bass extension or output. Without a subwoofer, they need more bass extension and output than even most full range loudspeakers offer.
Fifth is the practical aspects. For example, while large, two-piece systems like Pipedreams might be great speakers, placing 5 or more of them in a HT room might not be practical or esthetically desireable.
So, although many speakers that are considered good music reproducers would make good HT speakers, not all of them do. Conversely. speakers that offer the characteristics that make for good HT may lack the characteristics needed for satisfying music reproduction. If you need to combine a HT system and your primary audio system, there are some speakers and electronics that will do both well. I am in this situation, and feel that I've found speakers (by Vandersteen) that do both well.