I use to think like you, but experience tells me otherwise.
I dun recommend him to invest in advance cos:
i) Lens technology improve drastically ~ in no times new lens will superceed any current models;
ii) Lens prices drop rapidly these days ~ the EF-S17-55/2.8 is still relatively new with a high price-tag, which I believe, the depreciation on tis lens will superceed the loss when he decide to sell the Tamron 17-50/2.8;
iii) When one recognise his/her style/habbit, he/she will eventually turn to lens which are speciallizing in tat particular field. As we know that, there isn't any perfect lens tat covers all field/subject, one is better left reserve with choices till he/she very sure on his/her needs.