You mention cost as your only criterion, and others point out, correctly, that point-to-point advanced-purchase fares often add up to less than the price of a Eurail Pass. Others point out SNCF's pass-unfriendly system, which requires passholders to pay for reservations on high-speed (and, I should add, some conventional) domestic trains.
Given your desire to take day trips from Paris, don't forget the flexibility benefit of a pass. If you change your mind about which day trip destinations you want see, if you want to stay an extra hour or two before returning to Paris, if the weather is favorable/unfavorable on a given day, or if you're tired and want to travel the next day instead, a pass is your friend.
Even in the cases where passholders need to buy reservations for French domestic trains, those reservations (which have standardized prices of €10 and €20 where reservations are required, or as little as €1.50 in the few remaining cases where reservations are offered but not required) are easily and freely exchangeable until the last minute. You can complete the exchange at any SNCF ticket machine, without having to wait in line. This works even for paper coupons.
In France, discounted advance-purchase tickets may be non-exchangeable, or exchangeable only up to the day before travel, or exchangeable for a fee. The greater the discount, the greater the restrictions.
From your reference to 5 or 7 days, it sounds as if you know about "flexible" passes, which let you choose travel days within a bounded period of time. Sometimes, a flexible pass with a smaller number of travel days, plus some point-to-point tickets, gives the best of both worlds.
Last but not least, Eurail Passes are sold at a discount during certain periods of the year. You might be able to take advantage by buying in advance. Search online to find past sale dates (and upcoming dates, if you look really carefully!).
Be sure to consider your financial exposure in the event that you don't travel for whatever reason. This concern applies as much to passes as to point-to-point tickets!