Dan
Simple solution
Give us a limited number of tyres per season and make us use them as we feel fit, that way you will Soon use all your soft tyres meaning you have no choice but to use others.
Strategy comes into play more this way too as you will need to decide when and where to use a possibly slower tyre but were you can also use a strategy that maxes that tyre.
Too artificial of an idea, in my opinion.
Racing, in iGP Manager, real life, or any other racing game, should be about who is the fastest, in the given conditions.
We want to see managers/drivers pushing to the limit ('Maximum attack, flat out' as Mika Hakkinen used to say) not races decided by selecting the right allocation of tyres over the course of the season.