Yeah, I see absolutely nothing wrong with using Prato as a sort of stepping stone for our youth. As Browha points out, Inter seems to have serious problems with figuring out which of its primavera players are actually potential first team quality (see Longo for example). By giving our youth a situation where they can get consistent first team experience, we are not only giving them some valuable developmental experience, but also giving ourselves a wider range of performances to gauge their skill levels from, something that we might not get from loaning our players to a Serie B bench or from them playing against other primavera teams
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Short answer: yes
The long answer is that by playing against teams that aren't primavera you'll find that the playing style -even if not up to Serie A standard- is still similar to what these players will be seeing when they ultimately transcend out of the primavera level. Serie B and Serie C1, while not exactly similar to the Serie A in terms of technical skill, are still very similar to the Serie A in teh physicality of the game, something that I personaly think is a good thing to acclimatise our youth to.
Also I'd like to point out that Prato is a decently good C1 team, and with our help they actually have a good chance of reaching Serie B, in which case the skill gap will be even smaller.