As I produce about a swimming pool’s worth of sweat for even the easiest of sessions, a turbo-only bike is what I’ve used up to now. You’ll be surprised what deals you can get on eBay - my latest turbo bike is a Bianchi c.2000 which I got for virtually nothing because of a crack in the headset tube. I’ve had it for almost 5 years, although it is on its last legs now.
The only problem is the position. As a TTer, I know I ‘should’ be training in the aero position as much as possible, but don’t want to take my TT bike anywhere near the sweaty mess that is my turbo room. Fortunately, I’ve recently acquired a frame and some spare parts, so plan to build up a new turbo bike from scratch, and use clip on aero bars to get as close to my TT position as possible.
I guess it depends how much you sweat - however many towels I use, my turbo bikes always fall to bits eventually, so I’m reluctant to start using my ‘proper’ bikes on there.