Essentially, L0 gives you a grounding in all basic Athletics, including Swimming and Throwing, which will avoid a -3 penalty in many circumstances. Some circumstances won’t apply the default penalty as they are so common.
L1 in eg Athletics (Endurance) ADDS DIRECTLY TO YOUR ENDURANCE. ie your End. of 8 now counts as a 9 (and giving you the +1 for score of 9). L2 would add +2, etc.
This works great for STR, CO ORD (DX) and END.
For the very rare circumstances when the skill itself would still be useful (eg END for Hiking), I will still allow the skill level as a bonus: its not like this ever crops up much.
For Athletics (Swimming) – the only other one anyone is ever likely to take – L1 and above allows you to scuba dive, and will apply direct skill bonuses on all matters relating to Diving.
The downside is that levels of skill in Athetics add to your skill pool of course, and make future training harder. However, it will give a new lease of life to Athletics I feel.
Clearly this is different to the stated skill description in the book – but then again the skill as stated is basically useless: a wasted skill.