As a ham, we all tell people who are aspiring to be hams to "not push the PTT" when one's not licensed yet.
In this situation the same still applies, this is a commercial band frequency that was assigned to someone else, and while it still "works" perfectly fine, you're not licensed to use it and thus are against FCC rules and regulations, no different than amateur bands.
Agreed that likely nobody will bother you because each frequency is not constantly in use. However as a wideband, you could be interfering with more than one licensed user.
While I don't have a HT-90, I have another non PLL-synthesized ham radio that I'm trying to design/build a crystal eliminator for. As a ham targeting in the amateur bands, this is okay for experimentation and learning. However such homemade device is not legal for use in the GMRS/MURS/CB/... bands as the whole system needs to be type approved. As an additional food for thought, even if you get a crystal that locks these into the GMRS bands it's still illegal (though nobody may notice...) if it wasn't part 95 GMRS approved by the FCC.
All in all, if you aren't interested in hacking and building crystal eliminators and be restricted to the ham bands, agreed, these non PLL-synthesized radios are probably not worth anything these days and probably should be relegated to a museum... or destroyed if you can't obtain a license to use them