it's a no brainer isn't it. The top two get an extra ride (nominated heat 15, plus a potential TR) and they're in all bar 6 of the 15 heats so you need two top riders otherwise you concede 8 points across heat 13 and 15 and can't get a heat advantage in 7 of the other heats. arguably you could go for strength in depth to secure an 8 point advantage across heats 2 and 8 but as soon as you have a 'rising star' restriction or whatever it's been over the years, you nullify the strength in depth option.
I've said for years, if you restrict the reserves you got to restrict the top two, i.e. only one rider averaging over 8 or 9 whatever top men average these days. It makes complete financial sense as well, as demand outweighs supply but clubs that can afford two of them will never vote for it as it's a huge advantage.
The heat format is so regulated these days there's basically no point in turning up if you've not got two top riders. Especially when you can have guest riders for missing top riders so there's no jeopardy in putting all your eggs in the top two basket. Although Ipswich would argue that point as they'd have walked the title this season if they'd been able to replace Doyle and Sayfutdinov satisfactorily.