I think it's natural to think that way and I think many people feel exactly the same way.
You spent good money for full-range towers, darn it! So now you must use them full-range!
Well, even if we don't use towers completely full-range, I think we can still gain DYNAMICS and efficiency over most bookshelf/monitors (unless the monitors are big bad ones). So it may be possible to get better SQ using towers + subs vs monitors + subs.